Nano: how to obtain syntax highlighting

I have a default installation of nano under /usr/bin/, and I am trying to make nano highlight the bash scripting language code.

I have been searching around, but I had no success: I got always some errors, or simply doesn't work. For example, I have tried to download the following repository from Github github.com/nanorc/nanorc.git with the following command:

git clone https://github.com/nanorc/nanorc.git

and then follow the steps in this post, but I have had errors because some command inside the files are not recognised. In particular, if I add this line include ~/nanorc/shell.nanorc to my file ~/.nanorc, where the file shell.nanorccontains:

syntax "sh" "\.sh$" "\.bash$" "bashrc$" "bash_profile$"
header "^#!.*/(env +)?(ba|k|pdk)?sh[-0-9_]*( |$)"

FUNCTION: "^[A-Za-z0-9_+-]+[[:space:]]*\(\)"
KEYWORD:  "\<(case|do|done|elif|else|esac|fi|for|function|if)\>"
KEYWORD:  "\<(in|select|then|time|until|while)\>"
FUNCTION: "\<(echo|exit|export|let|local|read|return|set|shift|umask|unset)\>"
FUNCTION: "\<(source)\>"
OPERATOR: "[{}()<>!=&|;`$]|\\|\[|\]|-(eq|ne|gt|lt|ge|le|s|n|z)\>"
STRING:   ""(\\.|[^"])*""
VARIABLE: "\$\{?[A-Za-z0-9_!@#$*?-]+\}?|`[^`]*`"
STRING:   "'(\\.|[^'])*'"
ESCAPE:   "\\.?"
+HASHCOMMENT
+TODO
+LINT

It seems that these previous commands, such STRING, ESCAPE, etc, are not recognised.

How would you set syntax highlighting for nano, at least for bash?