Unable to Define Relationships on Reverse Engineered Model

I had reverse engineered our data warehouse schema into a data model using data modeler.
I am trying to redefine the existing relationships and re-engineer our current environment, however, I am having difficulty in defining the relationships between entities.

Our database currently does not "enforce" the relationships using foreign keys for the sake of saving some overhead on the database maintenance, rather the data integrity logic is built into the ETL.

We are using SQL SERVER 2017 Developer Build 14.0.1000.169 (Standard in PROD).

EDIT: Forgot to mention that we are using Toad Data Modeler v6.5.5.6

Please see screenshot below:

Hello,
it looks like your primary key "PK_Dim1_Claim_CSD" in the parent entity "Dim1_Claim_SCD" does not contain any attributes.
Normally if the parent entity has any attributes in primary key and you create relationship you can see this attributes in the column "Parent" on the relationship properties and foreign key attributes in the column "child". Also if you edit (F2) column "child" you can map an automatically created FK attribute to already existing attribute from child table.

If you want to use the attribute in unique index or alternative key as the source you can select it in the combobox "Parent key".

Regards
Dave