When I try a simple flow to create records after a record created I get a "Resource not found for the segment" error.

I've tried all the options suggested elsewhere.
Clients(GUID)
/Clients(GUID)
/Clients/GUID
GUID
I also tried c9_memberships(GUID) (which is the field name)

GUID on it's own, and c9_memberships(GUID) returns the error "Resource not found for the segment 'f5e89b62-36d9-e911-a813-000d3a86d655'."
all the others return the error "Resource not found for the segment 'Clients'.)
I get this for both the standard Common Data Service trigger, and the Common Data Service (Current Environment) trigger.
Shouldn't this just work with the GUID?
Any suggestions?