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