Why can't I apply this?

So I’m setting up a character for a third person shooter. However there is a problem. I’m trying to attach the weapon to the character’s hand however for some reason it sends the gun into the air. Another issue (Not sure if related) is that I can’t click apply to the changes I’ve made to the socket in the animation tab of the mannequin skeleton. Anyone wanna please help me out?

Many thanks in advance.

You can’t apply because you are using a "construction script ". For resolve the problem of gun you need to put in location rule “snap to target”

Skeleton changes are automatically applied iirc. Also, is the weapon moving with the arm, or just floating static above the player?

It moves with the arm

I’ve fixed the problem now. Thanks for your responses.