If you already have a WordPress installation and you want to change the table prefix, you have to set it into wp-config.php

You have also to dump your db, replace all wp_ with newprefix_ in the sql file, remove all the tables and restore the sql file.

Now check inside newprefix_usermeta and newprefix_options tables if there are ^wp_ values and change wp_ with _newprefix

SELECT * FROM newprefix_options WHERE option_name LIKE ‘wp_%’

SELECT * FROM newprefix_usermeta WHERE meta_key LIKE ‘wp_%’

https://www.wpwhitesecurity.com/you-do-not-have-sufficient-permissions-to-access-this-page-fix/