There are two likely reasons why your ETH balance isn't showing up in the trade window:
1. You didn't select a trading pair. Choosing a token to swap for your ETH with should make your max balance appear.
2. You don't have enough ETH to pay for the Ethereum gas and 0x protocol fees. Matcha subtracts the estimated gas + protocol fees from the displayed ETH balance. If you don't have enough ETH to cover these fees, the trade window will show 0 ETH. You can either add more ETH to your wallet or possibly wait until the Ethereum network is less congested, and gas is cheaper.
Check out these help articles for more information on the Ethereum gas and 0x protocol fees.