For any transaction to be completed, for example, deploying a contract on Matic testnet, Metamask requires some ETH to cover Gas fees. However, not everyone will have ETH in their accounts even if it's a testnet like Ropsten or Matic testnet.
To overcome this you can bring down the gas fee to zero so that your transactions can be processed.
Consider the example mentioned earlier, "Deploying a contract on Matic testnet"
When you deploy a contract on Matic Testnet, you will receive a pop-up alert from metamask to confirm the transaction, but it will show you an error that you do not have sufficient funds to complete the transaction.
Click on the Edit button in the pop-up and then chnage the Gas Price to Zero. You can then save this and then you’ll be navigated back to the original pop-up.
Now you won’t be seeing the alert anymore for, “Insufficient funds”. You can now click on Confirm to complete the transaction.