Why can't I withdraw GRAM (ex Toncoin) from staking?

The most common reason for withdrawal errors is insufficient GRAM in your wallet to cover the transaction fee. Make sure you have at least 1.2 GRAM available in your wallet before unstaking. Although the transaction fee is around 0.001 GRAM, due to TON blockchain specifics the pool mechanism temporarily reserves ~1.1 GRAM (although you keep holding them) and then returns the unused amount to your wallet.

Another reason could be a lack of liquidity if you selected the Instant withdrawal type. In this case, Tonstakers will send you an NFT confirming your withdrawal request ("Withdrawal Payout #X") and process it starting from the next election round (approximately 18 hours). You can check the status of your withdrawal requests in your Tonstakers account.