"Having four cats, I do not feel lonely."
Translation:Mając cztery koty nie czuję się samotna.
I think it is the same situation as here: https://sjp.pwn.pl/poradnia/haslo/Chcialbym-byc-bogaty-i-piekny;16960.html
(I asked that question, I wasn't sure myself.)
"Się" is movable so you can say "Chciałbym się nauczyć niemieckiego" as well as "Chicałbym nauczyć się niemieckiego" without much difference. But you shouldn't put "się" at the begining and at the end of the sentence if it is possible.
Here there is an additional rule that "nie" should be put before the word which it negates, in this case it is the verb "czuć" and "się" acts like an object of the verb.
It's the default sentence actually, so it's really strange if it wasn't accepted.
For example if you used "samotny" but made a typo somewhere, you would be corrected to the default sentence that chose "samotna", which could lead you to think that "samotny" is rejected, but this... this looks like a bug.