I can't figure out how to format the drive.
Sorry, we are confused. In your last post, you said that........ "I have tried APFS, exFAT, HFS+, MAC journaled....." You could not have already tried these different formats if you did not know how to format the external drive, correct?
Can you please provide some step by step details on how you formatted the drive in Mac OS Extended (Journaled)? This is the correct format to use with the Time Capsule if you are trying to back up to the external drive using Time Machine or store other files on the external drive.
If you connect the external drive directly to a USB port on your Mac......as a quick test......the drive icon should appear on the Mac's desktop.
Right-Click on the external drive
Select Get Info
Another window will appear with information about the drive
What is the listing under Format?
If you connect the drive to the USB port on the Time Capsule, does the external drive appear when you open AirPort Utility and click on the Disks tab? When you do this, you should see the Time Capsule drive........named "Data", unless you have changed the name of the drive.......and the name of the external drive if it has been formatted in Mac OS Extended (Journaled).