iCloud – Solving Appstore Error 500 (com.apple.commerce.client)

I've just formatted my SSD and reinstalled MacOS Mojave 10.14.3 (18D109). So this is a clean install. The first thing I did was try to install Keynote, Pages and Numbers.

The problem is: I cannot connect to my apple account through appstore and cannot install any app (I urgently need Keynote).

I found several people online with similar problems and followed some steps but all of them unsuccessful

I tried to sign out of icloud, restart, sign in

I tried to restart the laptop several times

I tried to clean some folder in the Library

I tried to remove the app-store configuration files

I tried to sign in on itunes and sign out

The current behavior is: I open the app store and see that I am not signed in on the lower left corner. Then I either try to sign in inputting my email and password but failing silently, or I try to install an app while signed out and it asks me to sign in. After inputting username and password the error 500 (com.apple.commerce.client) appears.

If I incorrectly insert my password, I expectedly see a message saying that the password is incorrect.

I would love to understand what the problem is and how to solve it.

Please help me!