It happens when your wallet doesn't have enough Eth. Please make sure you have at least 0.01 Eth in your Wings wallet.
In order to buy some Ether you can use changelly.com, as well as transfer Ether from one address to another. If you've got some Ether at some address you can just transfer it to your Wings address using services like changelly.com or myetherwallet.com. Otherwise one can just buy it for USD at changelly.com
This article shows how to use MyEthWallet service - https://blog.wings.ai/using-myetherwallet-with-wings-7b2db0424fd9