The separator plate gaskets between the valve body and transmission case have probably failed, causing a pressure leak at the reverse band servo. Or the reverse servo o-rings have failed. Not much you can do except to start saving for a rebuild or used trans. Common problem for the 5R55E trans, mine did it and I wound up swapping to a manual lol.
A trick is to shift into reverse and wait maybe 5 seconds and then touch the gas lightly, sometimes despite the leak it will build enough pressure to engage the reverse band and let it work, but it will probably slip like mad if you try to give it too much gas.