Logitech webcam C270 no longer works on MacPro

After using the camera for years with no problem, I had to change my OS downwards from High Sierra to Big Sur, and install a different graphics card. Since then, I've not been able to connect the webcam. When I log in to Zoom to try and start a meeting, the menu shows that Zoom recognises the camera exists, but when it comes to asking Apple Security to accept it on the Mac Pro, nothing appears in the prompt menu, so it can't work. Can anyone help?