Ok, I’m not exactly sure what you did (but I suspect that you cloned FunscriptToolbox source code and copied it into ‘extensions’ folder of OFS).
This isn’t the way to use FunscriptToolbox. I made a topic for each ‘use case’ of FunscriptToolbox. The link above is not the best for your use case. The feature for synchronizing funscript is not an OFS extension. You should look at this message (and the whole topic for more context):
https://discuss.eroscripts.com/t/funscripttoolbox/78653/10
If you still have trouble, let me know.