Admin password not working for sudo

I've been trying to run this command in the terminal:

$  sudo rm -ri ~/.Trash

However after being prompted to type in my password (which I believe it needs to be the admin's password) and after typing it, it just says: "Sorry, try again".

I am logged onto the only user account this Mac has, which has admin privileges. I've tried repairing disk permissions as well as reinstalling the OS (Mojave), but with no success.

Not sure what to do anymore! Can someone help me out with this one?

UDPATE #1

When I run $ sudo -l I get this:

Matching Defaults entries for diogopires on MacBook-Pro-DI:
    env_reset, env_keep+=BLOCKSIZE, env_keep+="COLORFGBG COLORTERM",
    env_keep+=__CF_USER_TEXT_ENCODING, env_keep+="CHARSET LANG LANGUAGE LC_ALL
    LC_COLLATE LC_CTYPE", env_keep+="LC_MESSAGES LC_MONETARY LC_NUMERIC
    LC_TIME", env_keep+="LINES COLUMNS", env_keep+=LSCOLORS,
    env_keep+=SSH_AUTH_SOCK, env_keep+=TZ, env_keep+="DISPLAY XAUTHORIZATION
    XAUTHORITY", env_keep+="EDITOR VISUAL", env_keep+="HOME MAIL",
    lecture_file=/etc/sudo_lecture, targetpw

User diogopires may run the following commands on MacBook-Pro-DI:
    (ALL) ALL
    (ALL) ALL

And if I try and run visudo or sudo -u diogopires visudo, I get:

visudo: /etc/sudoers.tmp: Permission denied

UDPATE #2

Furthermore, to check group membership, I got:

$ id -a | grep -o '[0-9]\+(admin)'
80(admin)

UDPATE #3

Upon running set -x, I ran and got:

$ sudo -l
+ sudo -l
Password:
Matching Defaults entries for diogopires on MacBook-Pro-DI:
    env_reset, env_keep+=BLOCKSIZE, env_keep+="COLORFGBG COLORTERM",
    env_keep+=__CF_USER_TEXT_ENCODING, env_keep+="CHARSET LANG LANGUAGE LC_ALL
    LC_COLLATE LC_CTYPE", env_keep+="LC_MESSAGES LC_MONETARY LC_NUMERIC
    LC_TIME", env_keep+="LINES COLUMNS", env_keep+=LSCOLORS,
    env_keep+=SSH_AUTH_SOCK, env_keep+=TZ, env_keep+="DISPLAY XAUTHORIZATION
    XAUTHORITY", env_keep+="EDITOR VISUAL", env_keep+="HOME MAIL",
    lecture_file=/etc/sudo_lecture, targetpw

User diogopires may run the following commands on MacBook-Pro-DI:
    (ALL) ALL
    (ALL) ALL
++ update_terminal_cwd
++ local url_path=
++ local i ch hexch LC_CTYPE=C LC_ALL=
++ (( i = 0 ))
++ (( i < 17 ))
++ ch=/
++ [[ / =~ [/._~A-Za-z0-9-] ]]
++ url_path+=/
++ (( ++i ))
++ (( i < 17 ))
++ ch=U
++ [[ U =~ [/._~A-Za-z0-9-] ]]
++ url_path+=U
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ ch=e
++ [[ e =~ [/._~A-Za-z0-9-] ]]
++ url_path+=e
++ (( ++i ))
++ (( i < 17 ))
++ ch=r
++ [[ r =~ [/._~A-Za-z0-9-] ]]
++ url_path+=r
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ ch=/
++ [[ / =~ [/._~A-Za-z0-9-] ]]
++ url_path+=/
++ (( ++i ))
++ (( i < 17 ))
++ ch=d
++ [[ d =~ [/._~A-Za-z0-9-] ]]
++ url_path+=d
++ (( ++i ))
++ (( i < 17 ))
++ ch=i
++ [[ i =~ [/._~A-Za-z0-9-] ]]
++ url_path+=i
++ (( ++i ))
++ (( i < 17 ))
++ ch=o
++ [[ o =~ [/._~A-Za-z0-9-] ]]
++ url_path+=o
++ (( ++i ))
++ (( i < 17 ))
++ ch=g
++ [[ g =~ [/._~A-Za-z0-9-] ]]
++ url_path+=g
++ (( ++i ))
++ (( i < 17 ))
++ ch=o
++ [[ o =~ [/._~A-Za-z0-9-] ]]
++ url_path+=o
++ (( ++i ))
++ (( i < 17 ))
++ ch=p
++ [[ p =~ [/._~A-Za-z0-9-] ]]
++ url_path+=p
++ (( ++i ))
++ (( i < 17 ))
++ ch=i
++ [[ i =~ [/._~A-Za-z0-9-] ]]
++ url_path+=i
++ (( ++i ))
++ (( i < 17 ))
++ ch=r
++ [[ r =~ [/._~A-Za-z0-9-] ]]
++ url_path+=r
++ (( ++i ))
++ (( i < 17 ))
++ ch=e
++ [[ e =~ [/._~A-Za-z0-9-] ]]
++ url_path+=e
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ printf '\e]7;%s\a' file://MacBook-Pro-DI.local/Users/diogopires

And afterwards, I ran and got:

$ sudo ls
+ sudo ls
Password:
Sorry, try again.
Password:
Sorry, try again.
Password:
sudo: 3 incorrect password attempts
++ update_terminal_cwd
++ local url_path=
++ local i ch hexch LC_CTYPE=C LC_ALL=
++ (( i = 0 ))
++ (( i < 17 ))
++ ch=/
++ [[ / =~ [/._~A-Za-z0-9-] ]]
++ url_path+=/
++ (( ++i ))
++ (( i < 17 ))
++ ch=U
++ [[ U =~ [/._~A-Za-z0-9-] ]]
++ url_path+=U
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ ch=e
++ [[ e =~ [/._~A-Za-z0-9-] ]]
++ url_path+=e
++ (( ++i ))
++ (( i < 17 ))
++ ch=r
++ [[ r =~ [/._~A-Za-z0-9-] ]]
++ url_path+=r
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ ch=/
++ [[ / =~ [/._~A-Za-z0-9-] ]]
++ url_path+=/
++ (( ++i ))
++ (( i < 17 ))
++ ch=d
++ [[ d =~ [/._~A-Za-z0-9-] ]]
++ url_path+=d
++ (( ++i ))
++ (( i < 17 ))
++ ch=i
++ [[ i =~ [/._~A-Za-z0-9-] ]]
++ url_path+=i
++ (( ++i ))
++ (( i < 17 ))
++ ch=o
++ [[ o =~ [/._~A-Za-z0-9-] ]]
++ url_path+=o
++ (( ++i ))
++ (( i < 17 ))
++ ch=g
++ [[ g =~ [/._~A-Za-z0-9-] ]]
++ url_path+=g
++ (( ++i ))
++ (( i < 17 ))
++ ch=o
++ [[ o =~ [/._~A-Za-z0-9-] ]]
++ url_path+=o
++ (( ++i ))
++ (( i < 17 ))
++ ch=p
++ [[ p =~ [/._~A-Za-z0-9-] ]]
++ url_path+=p
++ (( ++i ))
++ (( i < 17 ))
++ ch=i
++ [[ i =~ [/._~A-Za-z0-9-] ]]
++ url_path+=i
++ (( ++i ))
++ (( i < 17 ))
++ ch=r
++ [[ r =~ [/._~A-Za-z0-9-] ]]
++ url_path+=r
++ (( ++i ))
++ (( i < 17 ))
++ ch=e
++ [[ e =~ [/._~A-Za-z0-9-] ]]
++ url_path+=e
++ (( ++i ))
++ (( i < 17 ))
++ ch=s
++ [[ s =~ [/._~A-Za-z0-9-] ]]
++ url_path+=s
++ (( ++i ))
++ (( i < 17 ))
++ printf '\e]7;%s\a' file://MacBook-Pro-DI.local/Users/diogopires