Could you provide us the related error message if possible?
Are there any other owners in your Team?
In theory, teams owner have access to add member in Teams. So please make sure again you are the owner in your Team. You can also try to assign other member as owner if you are Teams admin and try to add user in your Teams to see if it works.
In addition, it recommends you contact IT admin to check if there is any related error message in Health status of Microsoft Teams from Microsoft 365 admin center > Health > Service health.
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