You can't select a seat associated with a fee unless you already have a seat in the same section and therefore have either paid the fee or are entitled to the swat due to status or fare class. On most airlines, even if you're entitled to a special seat due to status, if you don't have such a seat assignment, you cannot move there (unless there's a mechanism for paying the fee on board, with the exemptions for status).

Some carriers block seats and normally you cannot move into a blocked seat if someone objects.

Some carriers would also object to, for example, an adult moving next to a UM when other seats are available.