Look for "warning triangles" to the right of the input fields. They would indicate erroneous or missing paths, which probably is the reason.
Low-level flying — It's all fun and games till someone looses an engine. (Paraphrased from a YouTube video) Improving the Dassault Mirage F1 (Wiki, Forum, GitLab. Work in slow progress)
There was a warning triangle on the "executable" line due to an incomplete pathway. I was able to clear the triangle by completing the pathway with instructions found in wiki.