SCHUFA is a private credit bureau holding a file on nearly every adult in Germany. Landlords ask for a SCHUFA-BonitätsAuskunft (a one-page version made for showing to landlords) as proof you pay your bills. You can order one yourself; there is also a free version you are legally entitled to once a year, though it is deliberately less convenient and arrives by post.
If you have just arrived, you have no SCHUFA record, and no amount of paperwork from your home country substitutes for one. This is a large part of why the permanent rental market is effectively shut to newcomers, and a large part of why the sublet market exists at all. It is not personal and it is not something you can fix in your first month; a record starts accumulating once you have a German bank account and a phone contract.
The important part: a short sublet has no business asking for your SCHUFA. The main tenant is not extending you credit, they are lending you their flat for eight weeks. When a “landlord” for a short let demands a SCHUFA report, a passport scan and a bank statement before you have seen the place, that is not diligence. It is a request for a complete identity package from someone you have never met.
For a Nachmieter takeover, by contrast, being asked is completely normal, because there you really are signing a long lease. If you cannot produce one, an employer's letter confirming a permanent contract and salary carries real weight, and a Bürge, a guarantor, solves it outright.