Cant drag main camera into slot

Hello, i am very new to unity and have a small problem with my gun script in my game. As seen in the first image, to make the gun work you have to put the main camera into the fpsCam slot. Unity seems to recognise the main camera as a camera because when i click on the add button next to the slot the main camera shows up. the only problem is that when i select it and move it into the slot it doesn’t work and the slot is still empty. Please help.

6645448--758827--Gun script slot.png

Have you fixed all compiler errors? Look in log and make sure the red error button (upper right corner of log) says zero errors.