score:4
There are several options outlined on this page. The easiest way for a single ticket seems to be to buy a QR ticket online, or use the pilet.ee app on your mobile phone. (Update: I just noticed you need a credit card for the website, too)
When traveling more often, it would probably make sense to buy a prepaid Γhiskaart, a multi day Tallinn Card, or try to use your bank card.
If you don't have a credit card, and your Maestro does not work contactless, the options seem to be limited. I think you may need to go into a physical store that sells bus tickets (see the link above), and pay either in cash, or using your Maestro card. Adding credit to the Γhiskaart should be possible in the same stores.