time machine issues

Assuming you want to delete ALL the data on the disk you want to use for Time Machine, you can reformat it using Disk Utility in the Utilities folder. Go to Time Machine in System Preferences and start afresh from there:

Mac Basics: Time Machine backs up your Mac - Apple Support

Reformat the drive using Disk Utility/Erase Mac OS Extended (Journaled), then click the Option button and select GUID.