[2020] question regarding mod/weapon

Some mods are incompatible as they take the same 'slot' on the weapon, Or are directly counter to one another. An example is the various Barrel mod for guns. Or any of the 'blade' mods (Tempered, Serrated, etc); Blade mods also conflict with the club mods as one strives to make sharper, the other strives to make flatter and bulkier.

Also of note, make sure you have enough mod slots, shown on the left side- One slot is for colors only, and the other slots are by quality level. Q1 and Q2 have 1 mod slot, Q3 and Q4 have 2, Q5 as 3, and Q6 has 4.