We usually stay below Instagram's limits with auto follows, but we'll bump up against those limits sometimes.
If you try to use the app at the same time, you might see:
This action was blocked. Please try again later.
You just need to wait awhile before trying again. If it doesn't work after a few minutes, try in an hour or two. Alternatively, you can pause Gold Nitro, use the Instagram app as much as you want, then unpause Gold Nitro.