Relationship error cardinality

Dear Jens/Team,

Following the course I am trying to redesign my data models in a better way.

I have encountered an error when trying to set up a one to many relationship.

Currently, I use a calculated column formula to look up rates from a table of rates. I think I can replace this with a one to many relationship, as the rate key is unique in the rates table.

However, when I create the relationship I get the error "The cardinality you selected isn't valid for this relationship".

I think this should be one to many, but whatever combination a apply I get the same error message.

I have screenshots of the relationships, data model etc but don't know how to attach them here?

Thank you,

Martin