Invitation could not be sent

Sometimes you can get "Your invitation to {profile_name} could not be sent" when inviting profiles to your network:

You can see this message in two cases:

1. The contact was invited before but declined your invitation or you withdrew it later (After withdrawing an invitation, you won’t be able to resend an invite to the same recipient for up to three weeks - LinkedIn Withdraw an Invitation).
2. You have too many sent pending invitations.

In both cases, you can’t invite a current contact even manually!

To check how many sent pending invitations you have, please open this link: https://www.linkedin.com/mynetwork/invitation-manager/sent/

If you have more than 1500 -  withdraw them and keep the most recent 500 - 1000: How to cancel sent pending invites?