Hi Juan,
First of all, welcome to OutSystems community and I'm glad you are enjoying working with it.
Regarding your situation, your error seems to be related to the fact you're trying to update a user without passing its Id. In that case, the database cannot identify the record you are trying to update and therefore it raise that exception.
To get the User Id based on the username, you will have to have a query on the user table filtering it with the username variable that the user inserted:

Note the validation the if is providing to avoid updating a user record that doesn't exist.
After you make sure a user with that username exists, update the password from the user record you retrieve from the query and then use that same user record to have the update (the same user record that exists on the database and that as an Id):

In any case, you perhaps want to review your current approach since anybody can set the password of another user if they know their username.
Hope it helps.
Regards,
João