Back up drive asks to be erased

That is expected behavior on the first use of a drive for Time machine.if this is an established Time machine drive, or contains items you would like keep, you will need to probe further.

The fist time you run Time machine, it evaluates whether the drive provided, in its current format, can support all the data structures required to support backup data sets (like a database of all your old files). this requires Mac format.

Many drives ship from the manufacturer formatted for Windows New Technology File System (NTFS) which Microsoft considers proprietary. Microsoft has said they will sue anyone who writes that format without paying them royalties. Macs do not write that format directly.