First of all thanks for buying the game and I’m sorry you’re experiencing this bug! It will likely be tomorrow night before I’m able to test and fix.
I’m thinking perhaps that the turret is visibly highlighted but not actually selected. Can you try clicking on the turret and the weapon again (even though they are highlighted) and see if that enables you to equip the weapon?
Either way I’ll try to repro and fix the issue, I’m just thinking that might be a workaround that allows you to play normally.