Many PC users are reporting mouse buttons refusing to work in the game. Here are some solutions that have worked for others:
If you’re using a controller, unplug it. The game won’t let you use a mouse if a gamepad is plugged in.
If you’re using a Razer mouse, turn off the Synapse software.
Alt-tab out of the game and back. This will solve the problem for a while. Repeat whenever it happens again.