Hello cantalope-watermelon,
We understand you need to use iTunes in order to restore your iPhone after being locked out. We'll do our best to assist with this.
In order to update iTunes, you'll need to update macOS. We'd suggest checking any installing any available updates after you get backed up: How to back up your Mac - Official Apple Support
Use these steps to update the Mac: Update macOS on Mac - Apple Support or see: If you see "Software update is required to connect to your iOS device" on your iPhone, iPad, or iPod touch - Apple Support
Let us know if that doesn't solve it for you. If not, what happens when you attempt to update your Mac? Are you having issues locating the iPhone in iTunes?
Keep in mind, you'll need to put the iPhone in recovery mode: If you forgot your iPhone passcode - Apple Support
Lastly, if you were using iOS 15.2 or later on the iPhone, you aren't required to use a computer. Instead you could try out these related steps: How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support
Hoping to hear how it goes, take care.