CREATE FILE encountered operating system error 5(failed to retrieve text for this error. Reason: 15105)

I was able to solve the problem running MS SQL Management Studio as ADMINISTRATOR.

It's a Windows permissions issue. If you connected to your server using Windows Authentication then that Windows user needs permissions to the file. If you connected to your server using SQL Server authentication then the SQL Server instance account (MSSQL$, e.g. MSSQL$SQLEXPRESS) needs permissions to the file. The other solutions suggesting logging in as an administrator essentially accomplish the same thing (with a bit of a sledgehammer :).

If the database file is in your SQL Server's data folder then it should have inherited the user rights for the SQL Server account from that folder so the SQL Server authentication should have worked. I would recommend fixing the SQL Server instance's account's rights for that folder. If the data file is somewhere else and the SQL Server account does not have permissions then you will likely encounter other problems later. Again, the better solution is to fix the SS account rights. Unless you are always going to log in as administrator...

Right Click on File mdf and ldf properties -> security ->full permission