What it means
The transaction's nonce (its sequence number) has already been used: either an earlier transaction with that nonce was mined, or one is already pending. Very often the action you are retrying actually went through the first time.
How to fix it
Check the address on a block explorer first: the original transaction may have succeeded. If you set the nonce manually, stop, and let the wallet choose it. If the wallet itself is stuck on an old nonce, resetting its activity data (MetaMask: Settings, Advanced, Clear activity tab data) resyncs it.