If you get errors from Time Machine regarding the state of your backup drive (external or Time Capsule), about the only way to fix it is to reformat your backup drive. This will erase all of the content on that drive, including your prior backups.
Plug the external drive that Time Machine uses into your Mac.
Open the Disk Utility app (in Applications > Utilities).
On the left side, select your backup drive, then click Erase in the toolbar. If you are using Catalina or newer, select APFS as the Format and GUID as the Partition. You can name it anything you want.
Once that is done Quit Disk Utility.
Open System Preferences > Time Machine and click Select Disk. Click on your “previous” backup drive and then the “Remove Disk” button.
Click the Select Disk button again and choose your “new” backup drive and then click the “Use Disk” button. If you want an Encrypted backup for security check the “Encrypt Backups” checkbox and follow the instructions.
That is it, your Time Machine should stop throwing errors. If it continues to throw errors, it may be time for a new backup drive.
If you have a Time Capsule, substitute steps 1-4 above with these:
Connect to the Wi-Fi network that has your Time Capsule.
Open Airport Utility (in Applications > Utilities). Click on your Time Capsule then Edit in the resulting window.
On the resulting window, choose the “Disks” tab then click Erase Disk (Quick Erase Security method is fine).
Quit Airport Utility.