I wanted to update that I’ve identified and resolved the issue.
The Problem: The error occurred because I had copied an activity from a Cross-Platform (VB.NET) project into my current Windows (C#) project. This introduced a mismatch in expression languages (VB.NET vs. C#), leading to the compilation error.
The Solution: To resolve the issue, I recreated the activity manually in the current Windows (C#) project to ensure it adhered to the correct expression language. Once I did this, error stop showing.
Thank you again for your help!