This is most likely the user doesn't have permission to access the app. Please follow the steps below:
1. Please go to Bigcommerce Store Admin > Account Setting / Users.
2. Click edit on one of the users
3. Ensure the app Stock Sync is enabled under "Single Click Apps"
Stock Sync works on Google Chrome and Firefox.
However on Safari (need to disable Prevent cross-site tracking)
CANNOT be used on incognito