Restoring database to UNC path on local drive
Sounds like "access is denied" -  make sure that the SQL Server service account can access the share with both read and write (whether you can access the share is irrelevant). If you did not create a share named D that points to the root of D:, then your syntax is wrong - you need to use D$. However I recommend creating a share that is NOT in the root of a drive, and making sure that the SQL Server service account (again, not you unless you manually set the SQL Server service account to run as your Windows account) has read and write access. Then it would be:

MOVE N'test'     TO N'\\PC91\ShareName\Data\test.mdf',
MOVE N'test_log' TO N'\\PC91\ShareName\Log\test.ldf'


However I have to wonder why you would want to use a UNC path for this in the first place. It means your database files are more at risk because the share can be removed or become temporarily unavailable while SQL Server is running.