In order to prevent conflicts, Tweak Settings may not be altered while a cPanel & WHM update is in progress
I’ve experienced an error while accessing Tweak Settings through WHM
In this case first we need to confirm whether the upcp is running really or not :
ps axfwwwu | grep cPanel\ Update
If there is any process let it be completed, otherwise the issue is caused due to the file /usr/local/cpanel/upgrade_in_progress.txt created by failed/incomplete upcp process.
Just move or delete this file to access the Tweak settings.