I am the administrator of one of the CRM accounts, but I am unable to invite a user to it. Why is this?

I am the administrator of one of the CRM accounts, but I am unable to invite a user to it. Why is this?

It may be because the user has reached the maximum limit of org invitations. A user can have only 20 pending invitations at a point of time.

Sending invitations to users from different organization accounts
CRM users who are already part of a CRM account can be added and invited to multiple org accounts. They will be able to accept the invitation from another account without having to disassociate from your existing account. Read more about adding and inviting users.

Accepting or declining invitations
The user can accept or decline the org invitation within 7 days from the following places:
  1. Email: The email invitation that is sent by the host will have the option to accept or decline the invitation. If the user is part of other org accunts, those account names will be listed in the email. 
  2. Organization listing: The user can go to the profile icon and click Manage. The list of Invited organizations will be available under Organization Listing
Invitations sent from CRM Plus and Zoho One accounts or any CRM account that has Cliq integration or IP restriction enabled will NOT be listed in the Organization Listings. Invited users can accept the invitations for these type of accounts ONLY from the invitation email they receive.