If you are receiving that error even after creating a new user and assigning them the database previledges, then the one last thing to look at is to check if the users have been assigned the preiveledges in the database.
To do this log into to your mysql(This is presumably its the application that has restricted access to the database but you as a root can be ablr to access your database table via mysql -u user -p)
commands to apply
mysql -u root -p
password: (provide your database credentials)
on successful login
type
use mysql;
from this point check each users priveledges if it is enabled from the db table as follows
select User,Grant_priv,Host from db;
if the values of the Grant_priv col for the created user is N update that value to Y with the following command
UPDATE db SET Grant_priv = "Y" WHERE User= "your user";
with that now try accessing the app and making a transaction with the database.