Hi, apologies for the long delay, I've been away from the forum for quite a while.
Best guess on the popup is that the starting directory for the debug run profile may not be set - the BOS executable seems to not be able to find the game data files.
(Aside: up to now, I hadn't considered to use the Visual Studio debugger when working with FTSE, hence none of the debug project stuff being set up. Given the way FTSE works, and not having original source code to the BOS executable, I'm usually debugging at the assembly/machine code level anyway, and I've gotten more comfortable with using tools like Ghidra and x64dbg for that.)