I assume the error displayed in your browser is:
SqlPatchManager::runSqlPatch() - PdoDb - query(): Execute error. See error_log.
In the php.log, it is attempting to create the si_products_attributes table but this table already exists.
This update would only happen if the last row in the si_sql_patchmanager table has a sql_patch_ref value less than 255 but the si_products_attributes table already exists.
This inconsistency suggest your database is in an unknown state.
To remedy this, you should restore both database and SI version to what you had running correctly for SI 2011 at the time you upgraded to PHP 7.4. After doing this, check out the content of the si_sql_patchmanager table and verify that the greatest sql_patch_ref value is less than 255. Also verify that there is no si_products_attributes table.
If both these assertions are true, then access the master_2019.2 code that you have loaded on your system.
Hopefully this will run correctly. Let me know either way.