What was the configuration of the ACR that wouldn't accept the silencer?
If you already have a silenced weapon I don't think it will let you use another silencer on it. (I don't remember at the moment how the script looks that situation) Also try equipping a different barrel or sight and seing it it takes the silencer then switching the barrel or sight back.
When you equip the stripped version the menu asks if you want to reassemble, reassembly won't happen unless you put the pipboy away. Reassembly takes two hands
I had an LMG and ACOG. I switched barrels and scopes and it still wouldn't let me equip a silencer. I only had one silencer and it was not equipped. And I would field strip, click on the ACR, press yes on the reassemble menu, exit out of the pipboy and nothing would happen. I don't know what was causing this, and I am trying to figure it out.
After I fiddled with it for a while the menu popped up like it was supposed to. And it would work from then on. But if I reload the save before it started working it still happens. I didn't add a mod or change any existing mods. I played right from the new game onward.
If I tried to equip the silencer it said I didn't have an ACR equiped (And I did).
Edit: I found out why it was doing that. If you equip the ACR and try to put on the silencer (without exiting out of your inventory) and you don't have field strip in your inventory, it says you can't equip it.
After you equip the ACR, exit out of your inventory, and go back in to your inventory, the field strip option just appears in your inventory.
I'll post screenshots if you want.