All good! In most cases if your server player controller (What the abilities would be referencing) is returning Null, then it could be 1 of two things.
The first would be that the reference Is not being set, or its being set and cleared.
The reference is being set on the client, which will not replicate to the server. And/or the server is replicating its Null reference when you’ve set it on the server.
The ValidateCardPlayConditions function gets the server controller passed though so my feeling is on point 2, where the Hit actor may be getting set on the client. You can either pass through the reference to the actor you want to set to the server OR you can retrace on the server to get the reference and set this in the Play Card function.
Send over some screenshots of where your setting the reference if you want, and Ill take a look to see if anything stands out.