Currently, the value you’re getting from the URL parameter is a text, but the dropdown content type is ‘Package’, so there’s a type mismatch, hence why the expression is red.
You need to define an actual Package for the dropdown’s default value/
Assuming that Package is a datatype (not an option set), which appears to be the case, in the Default Value you’ll need to do a search for Packages, then in your constraints use the Get Data From URL to match the text value of the ‘a’ parameter to whatever text field it relates to, then select :first item.