Transferring data to new iMac

Hi wiliam241,

Please refer to: Back up your Mac with Time Machine - Apple Support.

You also may need to format your external disk first:

1. Go to Disk Utility (Finder --> Go --> Utilities)
2. In Disk Utility, select “View” --> “Show All Devices”.
3. Select the top level, non indented, drive in the external section of Disk Utility that matches the name of your external drive.
4. Click Erase
5. Name: any name you want
6. Format: Mac OS Extended (Journaled) if macOS Catalina or earlier, APFS if Big Sur.
7. Scheme: GUID
8. Click Erase
9. Setup Time Machine again.

Cheers,

Jack