hello,If you add a large surge of people or +pages to your circles within a 24 hour period, Google's system will temporarily stop you from adding additional people even if you’re under the total number of people you can have in your circles. You may also hit this limit if you excessively shuffle your existing circle memberships. Repeatedly removing and re-adding people to different circles can push you to this limit. In these cases, just wait a day and you’ll be able to add people again. Please be aware that there is a limit of 25,000 contacts in the Google Contacts Manager.  If the user has exceeded that number of contacts, they are seeing the message  

I would also encourage you to report this issue using "Send feedback" tool.