If you used the "Add to SFDC Campaign" flow and the program is synced to a SFDC campaign, you will hit this error. The help docs used to have this in there, but it looks like Marketo removed it . In order to get this to work, you'd need to break the sync, run the flow, and the re-enable the sync once it's complete.
use "Change program status" instead. This will add the lead to the program, which automatically will add it to the sync'ed SFDC campaign.