A lone rook rarely wins against a far-advanced pawn by itself; it needs the king. Use the rook to restrain, bring the king to the pawn, and take it.
The pawn is on the sixth rank with its king beside it, and the white king is a long way off on f4. Rook alone cannot force anything here: if it just gives checks, the king shelters next to the pawn and shuffles. The job of the rook is to restrain while the white king walks in.