I've found some issues in your code also:
- In function MakeExcelConnection, you have a variable named "_fileName" in which your argument is "fileName" and the private variable inside that function is "_filename".
Solution to the errors you encountered:
#1 and #2 try to change the Platform target to x86
#3. You should modify your
public static void FillAccessDatabase()
to
public static void FillAccessDatabase(OleDbConnection _connection)