OS admin can't backup

The service account (SQL) is a OS admin, but that does not allow it to do backup? the error message is
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Msg 916, Level 14, State 1, Line 2
The server principal "SRBSV\POSRESTEST" is not able to access the database "TRACKER" under the current security context.
Msg 3013, Level 16, State 1, Line 2
BACKUP DATABASE is terminating abnormally.
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being a OS admin - does it give any privilege in SQL Server?

what permission will this service account need to be able to do backup/restore?