How to activate account when error "account pending" keeps occurring?

I tried to add another user as a moderator and this error keeps occurring:

ACCOUNT PENDING: Your account is currently not active. An administrator needs to activate your account before you can login.

Here is one of the solution (not an exact)

Depending on your security plugin (I use All in One WP Security), I had to manually approve the newly-registered users in WP Security > User Registration.