Do you have an organizational account for ArcGIS Online? Are you the administrator?
Please check the total number of members and the available number of members (for each user-type).
Under Organization > Licenses > User Types, check the available licenses
The available user-types in your organizational account may differ.
ArcGIS Collector license is included (bundled) for Field Worker, Creator, and GIS Professional user types. Please ensure one of these user-types is available. In case any of these user types is not available, you will need to delete one user or change user type of one member to Viewer (or any other available user-type), so that you could make room for another additional user.
If the concerned user is already a member of the ArcGIS Online organization, you may change his/her user-type, as required.
ArcGIS Collector: Requirements
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