Successful currencies meet three criteria:
(1) they are in widespread use as medium of exchange
(2) they serve as unit of account
(3) they are a stable store of value.
Most sovereign currencies fully meet all three criteria.
What about Bitcoin?
As we know, only in some countries allows bitcoin as a medium of exchange, and we also know that bitcoin selling is also not always stable so we can conclude bitcoin has not met all the criteria to make it function as currency.