Problem

A desk looks free but cannot be booked

A desk without a reservation can still be taken, because it belongs to someone as a fixed desk, sits in an open import row, or is archived.

What you see

The day plan shows no reservation on a desk. It still cannot be booked for a member, or a day guest does not even find it in the app.

A reservation is not the only way a desk can be taken. Four reasons take a desk out of the bookable pool without any booking being visible.

The desk is somebody's fixed desk. It is blocked from the start of the assignment, which is often months before that person first shows up. It is blocked on every day the assignment runs and the member holds an approved contract with a fixed desk.
An open import row names this desk. If you uploaded a member list with a desk in it, the desk is blocked from that row's contract start. The person has no account yet, so the desk belongs to nobody you can find in the member list. It is refused to everyone all the same.
The desk is archived. Archived desks are neither bookable nor assignable.
The member holds a fixed desk that day themselves. Then they book nothing else, neither a second seat nor a place in an anonymous area. Their own fixed desk is theirs all day anyway.

Open the desk in desk management and check whether it is archived.
Open the member profile of the current holder and check the fixed-desk assignment. You can end it or move it to another desk. See Assign a fixed desk.
If you find no member for it, look at the open import rows. The desk on an import row can be changed or cleared without uploading the list again. See Import members.
If it is about a member who holds a fixed desk themselves: for one day at another place you swap the desk instead of adding a second reservation.

Check the audit log for when the assignment or the import was created. If the app does something wrong: report a problem.