Mac storage basics · Time Machine basics
Can I use a USB flash drive for Time Machine?
Short answer: Technically yes, but flash drives are usually small, slow for backups and wear faster. Use a dedicated hard drive or SSD of at least twice your data.
Technically yes, if it's big enough, but it's a poor choice: flash drives are usually small, slow at the many small writes backups involve, and wear out sooner. Your backups would keep very little history.
A dedicated external hard drive or SSD of at least twice your data is a much better Time Machine disk.