Can't select file for Write to File action

I'm guessing I'm missing something simple here—apologies for the dumb question, and thanks in advance for any advice!

I have a macro that worked just fine when the file it targeted was in my Documents folder. However, I now need to target a file in an app's /Users/ryanjamurphy/Library/Application Support/com.app.something directory—but when I go to select the file for the "Write to File" action, it is greyed out. If I use brute force/ignorance and hit the "Save" button in the dialog, KM prompts me with a "File already exists... replace?" warning. Yet, even then, the macro fails.

I'm guessing this is a permissions issue? I gave KM and KM Engine both full disk access, but can't think of what else I'd need to do to say "yes, please, have at it!" to KM.

(The file's a .css file, and it's for toggling a CSS feature in Obsidian).

I'd still like to figure this out, but I thought of a workaround: write to the file via AppleScript. So, I put the text I wanted to write in a variable and used Execute AppleScript to achieve the same function.

I'd still love to know why KM can't write to my Application Support files, even with full disk access... maybe I just need to reboot?