A lot of our customers claimed they got this popup every week after about 100 invites. According to many LinkedIn-related websites, as well as LinkedIn support representatives, a new limitation will soon be placed on every profile in LinkedIn, capping the invitations to about 100 per week - Invitation Limit Reached - LinkedIn
Currently, not every LinkedIn account is under the limit since some of them are still able to send more invitations, but it seems that LinkedIn is testing new limits on a part of its users and this is a matter of time when they are implemented for everyone.
There is nothing to worry about as this is not a warning or a restriction that may lead to an account ban. LinkedIn has been always fighting against platform abusers, so this is just another attempt to protect their users from spam and maintain their reputation as a professional platform.
In this case, if you hit the limit, the only thing to do is to wait till the next week as it is not possible to get more credits even if you have a paid subscription or you have withdrawn sent pending invitations or some of them were accepted.
We suggest as well, to reduce the number of daily invites per day to warm up your account once you are notified with the weekly limits.