On the underside of the bbl, there is a small rectangular piece of metal that engages the forend iron and it should move back and forth easily.
If it's stuck, the forend won't go on. Put some oil on it and try it!
The bbl needs to be off of the receiver to check this.
If that's the problem, then it's probably a small "burr", etc which can be fixed easily.
If it's not that, then it's too tight and can be adjusted quickly by someone that KNOWS what they are doing.
Good luck