Cannot log in to the MySQL server

I enter my control panel, go to phpMyAdmin and another page opens where it asks me for a username and password, I already entered the username and password for the control panel, for the database and nothing works... In theory that window should not appear, I would just log in directly and that's it.

Make sure your account isn't out of free space. If it is ok then try to reset your CWP password and then make new attempt to access MySQL using PHPMyAdmin.

Are you able to login as root okay? Is it just end users who cannot login to phpMyAdmin?
Are sites still working that use various DB users (e.g. kostivca_wp1234)?

I have already put all the passwords, root, client account, database user and password, and nothing works.

Make sure your account isn't out of free space. If it is ok then try to reset your CWP password and then make new attempt to access MySQL using PHPMyAdmin.

I don't have any extra space

Clean log files to gain space:
https://blog.alphagnu.com/how-to-clean-all-log-files-in-cwp-control-webpanel-to-gain-disk-space/
Search for other big files:
https://blog.alphagnu.com/how-to-find-big-files-in-linux-cwp-centos-ubuntu-debian/