Hi Lisa,
Can you provide us with repro steps, along with specifics on where you are seeing this?
Was Litmus working previously?
Are you trying to send this out of Prod or a non-prod environment?
Can you doublecheck the technical workflows and make sure they are all running?
Can you check the web.log after reproducing the issue to see if we get any more details about the error?
If your environment is hybrid or on prem, can you enable verbose logging on the web module and reproduce the issue (this should provide more details).? If hosted please reach out to Campaign Support.
Thanks,
Craig