How to submit German tax return from overseas?

In most cases, only people who live in Germany (and thus have an address here) need to pay German taxes and thus submit a tax declaration. You seem to belong to one of the very few exceptions. (Maybe you can explain more about your situation?)
The various tax softwares (which are written by independent commercial organisations, not by the tax authority) are mass products and cannot deal with niche cases.
You better contact a German tax adviser (or the tax authority directly, but they are often less than helpful) on how to proceed. I am sure there is a way, most likely preparing the forms manually and mailing them - that always works!

Thanks for getting back to me @beppi
I worked in Germany for a month in 2020. My employer put me under class 6 because I hadn't received my ID yet by the time I was paid. So, I hoping to receive some Euros back since I paid quite a bit off of my paycheck. I also read somewhere that it's mandatory for class 6 holders to file a tax return (and I don't want any trouble/surprises in case I return to Germany one day).

Class 6 is the least favourable for the employee, that's why they put anybody with unclear status into it.
To be sure whether (and how) you must submit a return, ask a tax consultant.
If so, it will be very complicated, because you have to submit your entire world income for the complete year - they will only tax the month in Germany, but the applied rate depends on what you earned elsewhere as well (that's called "Progressionsvorbehalt"). You'll not manage this without a tax consultant (again, this is not covered by standard tax software).
A quick check assuming you earned €3000 and had no family in Germany showed €400 possible refund - hardly worth the effort and the tax adviser's fee is probably higher.

Given I'm no longer in Germany (and only lived there for a month) would it still be mandatory to file a tax return being on class 6?

As I said, you'd have to ask a tax consultant for the (more complicated than it appears) question of whether you must submit or not.
But I guess (without warranty) that they won't pursue you if you don't.