Error after upgrading from 2.1.2 to 3.1.13

Hi everyone
after upgrading my app from version 2.1.2 to 3.1.13 i get some errors which don't make sense for exemple this one:

Code:

An uncaught Exception was encountered
Type: Error
Message: Class name must be a valid object or a string
Filename: /Users/mac/www/nephrogest/system/database/DB_result.php
Line Number: 205

my PHP version is 7.4 and i'm using MySQL 8
plz any help