So I really made a mistake by importing address without knowing I will be asked for a private key later since that address is watch-only. Seems like this address has already been used before on my account, but I have no idea is that mine or from someone else or mine from another site.

My question is: where can I check if I actually have private key to that address, hence where are they normally stored and do they have a life duration?

Also, every time I try to scan QR code, I am always getting an error: unsupported key format. But the system itself is asking to scan the code, not to enter the numbers. I have no clue what could be the issue except that the code is not supported.