Proofing feature not working for sheet Admin
I'm the workspace and sheet owner of a business plan account in Smartsheet. I've built a sheet on behalf of a manager who wants to utilize the Proofing feature to gather feedback from his team. I've shared the sheet with him and have given him Admin rights, however, he is shown a call out to upgrade his account (subscribe to Smartsheet) in order to unlock this feature. When we try to work around this by having me - the sheet owner - upload the document to the proofing column, he is not able to utilize the 'Request Feedback' button.
Why is this, and how can I correct it?