If it is the first time you are using the bridge you need to approve the bridge to spend your POLIS. A blue Approve POLIS button will show up. Click it and sign the transaction when the Metamask prompt appears. Execute the transaction and wait for it to confirm, after a couple seconds, the bridge will update and allow you to select the bridge button. If it does not refresh automatically after a couple minutes, please manually refresh your window, and you should now only see the Bridge button.