Computed Entity in dataflow with Premium per User not working

I have recently signed up to the Premium per user trial.

I have tried to amend one of my dataflows to use the fuzzy merge feature as I thought the linked and computed enitites are supported by the Premium per user plan.

But am getting the following error when trying to refresh the data

Error: On-Prem execution not supported for entity '179bfc53-ec0c-49fb-87ba-25a54bef9c45/8881b201-5205-4050-aeb7-ef8326b2a1fe/Proj_Documents' since it refers to another entity... RootActivityId = 847db981-9ecb-4435-8ac2-5e3d04520866.Param1 = 179bfc53-ec0c-49fb-87ba-25a54bef9c45/8881b201-5205-4050-aeb7-ef8326b2a1fe/Proj_Documents Request ID: 0b61b6a8-1066-be66-75c8-a5b3311f2479.

Any ideas?