iTunes backup button greyed out

The backup button in the summary page of my iPhone on iTunes is greyed out. There's plenty of storage space in the laptop, and the backups work with other accounts. I'm working with an iPhone 8 running iOS 14.5 and macos 10.12. other iPhones aren't working either.

Typically on Sierra, the first time you connect a phone running iOS 13 or iOS 14, you're supposed to get a prompt telling you that there's a minor software update that iTunes needs to do with a button to accept it. Do you remember seeing anything like that? You may need to unlock your iPhone first before the Mac realizes it's there and prompts you about it.

If not, here's an apple discussion thread about how to force that little update to happen - curious to see if that helps: https://discussions.apple.com/thread/250674376

According to that thread:

In Finder use the menu item Go > Go to Folder..., copy the following text, paste it into the dialog box and press enter:

/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/Current/Resources

Unlock your device and connect it to USB. Double click on MobileDeviceUpdater in the folder that has opened. It should notify you of a software update. Install it and iTunes should hopefully recognize your device.