You must always have SOL in your wallet to do any transactions. The recommended minimum amount to keep in your wallet is 0.05 SOL.
Gas is a network fee for sending a transaction, and every transaction on Solana will need some SOL.
There are also instances where the Solana network is congested, and may be difficult to confirm a transaction. This is usually during times of low TPS. You can view the TPS and general health status of the Solana network on the Phantom status page or the Solana status page.