He explained in his post:
He's saying add "$config['Misc']['debug'] = true;" (no quotes around the line) after "<?php" in the config file.
Yep, I knew there was another way but couldn't remember the link and was too lazy to go figure it out. And, I believe Opserty posted the easiest method to do this.

I have never heard of "restoring bitfields" are you sure you read it correctly? I haven't come across an option or anything of the like. Maybe you need to upload some bitfield_..xml fields or something?

It would probably be best for you to post in the modification thread for more precise assistance.

Finally, it is not recommended that you enable debug mode on a live site for long periods of time.

If you do not wish to remember to add index.php?do=buildbitfields to your url
here is a neat little XML file which adds "Rebuild Bitfields" under vbulletin options.

Just upload it to includes/xml

I cannot remember where I found it or I would give credit,
but I use it at every vbulletin forum I administer.
EDIT- I believe that I found the source:
Made by Yarub
https://vborg.vbsupport.ru/showthread.php?t=103565
It is from way back in version 3.5.3 but is still up to date for all new versions.
Attachment 82087

There really shouldn't be any reason to rebuild your bitfields.