The first possibility might be the reason for all this difficulty. With y53, one you click on Storage it will give you stats for both internal and SD like how many GB is available and how much is left. The closest we can get to "format as internal" is "Manage SD Card", however, when we choose that, we can only delete, share or compress the files that are already in there.
- From a user whose internal mem is at 98%
Saga Lout :
I removed your e-mail address to save you receiving a huge amount of Spam when the address has been sold a few times.
Two possibilities. Your phone manufacturer or Service Provider doesn't allow the making of that choice or the phone isn't running Android 6 or newer.
Another thought - if the three dot button doesn't show, you may have access to the menu at the bottom of your phone - below the screen and usually on the left side..