Serpentine belt installation issue

You don't have many options other then to try and hammer a smaller socket onto the bolt or replace the tensioner. I would not try and fix this but I would replace the bolt or the entire tensioner.

If you are going to replace it then use a set of vice grips on the bolt and that will grab into the metal and you should be able to extract the bolt.

I attached the process for all this below. Let us know if you have other questions.

https://www.2carpros.com/articles/serpentine-belt-tensioner-replacement