R Script permissions error
I think you 'might' need to grant the proper authority on that folder to the Windows group SQLRUserGroup.  

I have a test SQL Server instance where R-Services is installed.  I took your script and changed the directory to be C:/Scripts (just as a test).

EXECUTE sp_execute_external_script
       @language = N'R'
      ,@script = N'OutputDataSet &lt;- data.frame(file.access("C:/scripts/", 2));'
WITH RESULT SETS (([DefaultLibraryName] VARCHAR(MAX) NOT NULL));
GO


I got the same -1 return code as you.

I then granted FULL access against C:/Scripts to the Windows group SQLRUserGroup and ran the same script code.  It executed successfully with a zero (0) being returned.