The sheet I am trying to share is not in a workspace and the individual I am sharing to already has admin access. Im unsure what to do next since I have shared other sheets with this person with no problem.
It sounds like the other user may no longer have a paid license with Smartsheet, or has reached their max number of sheets that they are able to own, based on their plan-type. (You can read more about transferring ownership in our Help Center, here.)
User can be shared to sheets (with Admin or lower permissions) as a Free user or if they have reached their limit on sheets owned, but they will not be able to accept ownership of any new Smartsheet items.
If you don't believe this is the case, please have this other user reach out to our Support Team, providing their email as the point of contact so that our Support team can discuss account details with them.
Hope this helps!