OMG HOW do I buy a Deutschland Ticket on DeutscheBahn? As in.....SEPA Direct Debit?

Buy the Deutschland ticket from the Bremen Transit Authority (VBN) by downloading their Fahrplaner App, which accepts credit cards. Seat61 has step by step process.
https://www.seat61.com/train-travel-in-germany.htm#Deutschlandticket

I have no advice beyond what you have been given, but just to answer your question: "SEPA direct debit" is not related to a debit card at all -- it is a bank transfer system that we have in the much of Europe, if you have a Eurozone bank account, that lets payments be directly withdrawn from your account. So, there would not be any way for you to do this if you do not have a German Giro account or similar. I do not think any non Eurozone countries participate in this system.

"I DID buy my Eurail-Germany pass."

I assume you mean the German-Rail Pass, which I further assume will cover all your major travel legs within Germany.

I would not be terribly concerned about getting or not getting that Deutschland-Ticket. The GRP does not cover bus, subway, or tram travel. So in Berlin you will need to look into the options from the local transport authorities. An MVV (Munich) ticket or day pass, for example, will cover all different modes of local transport and is not terribly expensive. Cost depends on the distances you travel, but individual and group day passes make multiple journeys per day an option.

You should note however that on the same days you travel into and out of those larger cities using your German Rail pass, you CAN use the Berlin S-Bahn system or the Munich S-Bahn system for no additional charge. Within the city, you may end up making some longer walks, as the system is meant for traveling longer distances across the city and stops tend to be quite far apart; the other transport means are better suited for more focused sightseeing.

From Willaringen to Bad Saeckingen and back, a local day pass for two covers the direct round-trip bus trip at a cost of €11.

My recent experience- I used MVV app and website.
1) download MVV app and register
2) use a PC browser/MVV website for actual purchase. You will get error messages as you go through the process. Simply click “back arrow” to previous screen and then enter again. Somehow it works fine on second try.
3) US credit card worked fine.
4) Ticket shows up in the MVV app under “my tickets”
5) You receive an email confirming your purchase with the all important link for canceling your subscription. Hold on to this email.
6) cancel your subscription by the 10th of the previous month using the link in email. You may get error messages again. Simply go back a screen and try again it will work.

I purchased 3 different D-tickets for our party and it worked just fine. We only need them for the month of September so I waited one day and cancelled the subscriptions effective Sept 30th. You do need a different email address/account for each ticket.

I'm currently in Germany and used the Bremen AP fahrplaner. It worked very well using a Visa card. We've been using our Deutschland ticket and it has been great. Just remember to have your passport or drivers license with y.oh when you use it. Thank you Marred for your invaluable advice! We've been here a week and three more weeks to use it.

Even a little more comfortable is the app "Mainzer Mobilität" (and both much more than the MVV app mentioned above). Unlike the Bremen "Fahrplaner", you can cancel the ticket directly in the app and don't have to keep the email you received with the purchase, which contains the link to the cancellation.

(If your zip code is alphanumeric: type it in an editor and copy it into the corresponding field of the app, because that is only designed for numeric values).

BTW: do not try to buy the DT from Deutsche Bahn. This would only work with direct debiting, but they reject any foreign account, even if it has an IBAN. And while all the other carriers I've tried provide the DT on time in the app, DB likes to take its time, as is evident from the numerous complaints.