You cannot log into a SQL Instance to run commands on the database (when locally on the DB system) because you are not a SQL administrator - but are a system administrator.
First seen in
Assuming the user is an administrator on the system, this can either be because of:
- Their account has explicit permissions
- SQL 2008+, the default is only the installing user is a DBA. If that user is no longer around, it can leave an orphaned instance.
What To Do
To recover access to a SQL Server instance see the article below which details how to use Microsoft SQL Management Studio (SSMS) and the Microsoft tool 'PsExec'.