Unfortunately, the Catalina operating system has broken Time Machine for a number of users. In order to try to get things going again, you are going to need to perform a manual procedure to completely erase the Time Capsule disk and then try to run another backup.
On your MacBook Air.......
Open AirPort Utility (Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the Time Capule
Click Edit in the smaller window that appears
Click the Disks tab at the top of the next window
Click Erase Disk

When you click Erase Disk, a selection box will appear to allow you to choose the type of erase
Select the Zero Out Data option
Click Erase

The erase procedure is going to take a very long time.......up to a day or two. Let the process run until it completes. The Time Capsule will blink amber during this process.
When the erase procedure has completed........Restart your Mac and try to run another backup. This will also take a very long time.
Report back on your results.