Validation failing on a lookup field

Hello,

I have a Look up field for which I have custom validation set on that field as mentioned below:

if ("{!opportunityLookUp}" === "")
{
      return "Please select an Opportunity.";
}

But the above mentioned validation doesn't work on Lookup fields. I cannot use the Rollbase required field option because I am hiding and showing that field based on a certain checkbox selection. Could anyone suggest a solution for this issue please?

Cheers.