When you have too many unanswered invites, LinkedIn may temporarily restrict your ability to send new invites.
Step 1: Try sending a few invites manually on regular LinkedIn (not through Sales Navigator).
If it requires you to always insert the receiver email, pause the the invites ("I") of the Growth-X campaigns for several days for the restricted user.
Step 2: Withdraw older pending invites from your Manage Pending Invites section in LinkedIn that are over 2-3 weeks old. It should get to a point that you only have a few hundred pending invites remaining.
Step 3: Wait 48 hours from the moment you withdraw invites. After 48 hours, try again to send a manual invite to 5-10 people.
If you are still always required to insert the email for all receivers, let us know and we'll investigate further.
Instead, If sending invites manually worked, you can re-enable invites ("I") of your Growth-X campaigns.