Query input must contain at least one table or query error
I think you've got your VALUES clause a bit mixed up.

Is this what you're after?

INSERT INTO Grade 
       (StudentID, CourseID, Semester, Year, Grade) 
SELECT s.StudentID, c.CourseID, 'Fall', '2016','B'
FROM Student s
JOIN Course c
ON s.StudentId = c.StudentId
WHERE s.StudentID = 5812356
AND c.CourseID = 'CS101';