What message do you receive? What they say in that popup will tell you how you need to proceed. So far I've gotten 2:
1) You have followed too many people, please unfollow users if you would like to follow more.
What isn't told is that this type of limit is highly based on how many people follow you. It comes up when you follow a large amount of users but have very few followers or a very low % of followers in return.
For example, when you first start you can most likely follow up to 400 people, and most likely you will get that limit and have to unfollow people in order to follow new people. This is normal, and is part of starting and growing a pinterest account without any experience. Unfollow users, then follow new users again (rinse and repeat).
As you scale, as long as you have a decent follow back % (for example, at minimum 30% of the people I follow will follow me back), you can keep it automated all day as long as you have enough lag time (I do 2 minutes, which = to 30 followers per hour) and run it all day.
2) You are following too quickly and you have a ban.
This is pretty much what it sounds like. You are following people too quickly (a few seconds in between followers), and is considered follower spam. Simple solution is to wait 15-30 minutes and it'll go away and you can start following again. It sucks if you have an automated follower bot, because it can keep on trying and it wont get anywhere and most likely reset the timer.
To avoid this, increase your lag time in between following users. As stated above, my lag time is 2 minutes in between followers, and I can run it 24/7 (which I do) - but its really based on your situation. You can do more or less.