Why can't I donate using a credit card?

Actually I use a debit card for all my on-line purchases.

Edit Di 7. Mai 18:07:30 CEST 2019 (Opaque) - Moved addition from answer to this question and deleted answer, which wasn’t an answer but an additional comment

I donated previously, years ago, using a debit card, so why not now?

You can - but you need to select “One-time donation” and the credit card option appears.