How to set Validation Rules on Lookup Fields???

I cannot find the option to setup a Validation Rule on a Lookup Field.

Here's my use case:

We're in Real Estate so we sell Homes/Units
We have a custom module called "Units"
On the Deals/Potentials, I have Lookup Field called "Unit Selected" that looks up records from the custom module "Units"
When sales users go to move a Deal/Potential to a specific stage (e.g. Closed Won), I need a Validation Rule to enforce that the user enters a Unit in the "Units Selected" lookup field.

I am attempting to create a Validation Rule that validates that the user has specified a value in a Lookup Field on the Deals/Potentials object. When I go to setup the Validation Rule, I do not see any option for picking any Lookup Field to Validate.

Has anyone experienced this before? Is this functionality even possible?

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