Before you can stake your Flash you will need to get or rather give approval to the dapp to use flash tokens from the wallet that you connected.
Sometimes, or unfortunately quite regularly, the network is rather clogged which requires high gas fees. And the suggested gas amount from e.g. Metamask can be way off.
Best is to check https://ethgasstation.info/ and use a higher suggested number of GWEI.
In case your first attempt does not get through within the approval time of the Dapp, DO NOT initiate new approvals.
You will clog your queue until the first attempt is cleared.
How to solve is described here: