Can’t put my phone in the lost mode

My iPhone was stolen a few minutes ago and now I seem to be unable to mark it as lost in FindMy. Previously I could track my phone there but now the device appears to have been deleted. This prohibits me from marking it as lost and erasing it. Is there a way around this? Did I simply not enable it beforehand although I have tracked it in the past? And is the thief able to access my data somehow and my banking apps which are accessible by FaceID? I appreciate any help!