I'm just wondering if you need a valid driver's license before entering 31B training or can you get one afterwards?

You have to have one before because there are times in training that you have to drive, like during the law enforcement training where you drive a patrol car and respond to simulated calls.

MPs don't use Uber or Lyft, so yes, get a license.

It would behoove you to get one before training.

You need to get one before you go so you can drive during training.

If you're already in the Guard and have another MOS then you're looking at re-classing, if I'm correct. In that case, you will attend a USAR or ARNG 31B Phase 1 and 2 (2 weeks each) and without a valid civilian AND military license, you will not begin training.