can't attach mods

[Sir Green Day]

Here's what happens.

I go to the mid screen. I choose the weapon I want to mod. I choose "attach mods" . Then the next screen is blank. I know have the needed mods in my inventory and I know I can attach them because the icon is lit up under the picture of the weapon.

Why isn't it working?

[0sedax]

You have to unequip the weapon first.

[fang1192]

Unequip the weapon from all loadouts, attach mods through the salvage matrix.

[winterforge]

You have to make sure of the slot also. The gun will look like this:

[ ] [ ] [ x ] [ ]

You have to make sure the mod you buy matches where that X is (it won't be an actual X, it will be a small scope icon etc).