There are two possible reasons preventing withdrawal of funds from the miner:
The minimum withdrawal amount is 5 USD (or equivalent in BTC). Make sure your miner balance has enough funds.
The mining process is still active. Withdrawal is not possible while mining is ongoing. If the miner is still running, wait for the active session to end, then click "Withdraw".
Please note: to withdraw funds from the miner, you need to verify your email address. Instructions for email verification can be found here.