While technically correct, I suppose, I think most people would say "There was white carpet on the floor," meaning the floor was covered in white carpet. If it was only covering a small area, they would say "a white rug". I can't think of any situation where someone would talk about "a white carpet."
I put the white carpet instead of a white carpet. Thst should be acceptable. I reported it, please give me some feedback.
Yes, you are correct. It should be accepted. "A white carpet", "the white carpet" and simply "white carpet" should all be accepted.