Summer 2019, I had (have?) this problem with my Pixel XL. My "Use USB For" is greyed out. (I'd *like* to transfer files to my computer.)
I'd tested multiple cables on multiple computers. (including cables & computers that used to work previously)
I had tried rebooting the phone.
A couple of weeks passed, and I tried again just now. Same problem. But this time, when I rebooted my phone, the problem immediately resolved itself!
(may be worth testing: I rebooted the phone while it was plugged in to the computer... not that this should matter.)
Anyway: My conclusion (based on meager evidence) is that rebooting *may* fix the problem. Rebooting a couple of times may also help.