All set?

If you've finished signing, we need to verify your bank account before you can complete your purchase. We’ll make a very small deposit into your bank account, so keep
an eye out for it. Depending on your bank, it can take 1-3 days.

The deposit you're looking for will include the word "Microsoft" in the description. Note the deposit amount and then:
  1. Go and start your purchase again.
  2. When you see your Direct debit account, select Verify.
  3. Enter the deposit amount and select Next.
  4. Complete your purchase.