I'm not disagreeing with you, but it's a good thing to know that this is the equivalent Polish construction to "there is/are"
"there is/are" + noun + preposition + place
preposition + place + "być" + noun
"Kubek jest na stole"
The mug is on the table"
"Na stole jest kubek"
"There's a mug on the table".
At least that's how I understand it.
I have used 'boot' instead of shoe often before so why has it been rejected this time? I have always used and seen buty as either shoes or boots. Duo does not give me the option this time of saying it should be accepted. There is nothing implied in the context to suggest that one cannot use 'boot'.