If this "(D:" is not a typo and is indeed what was executed then that error return is correct:
ALTER ASSEMBLY ClassLibrary5
FROM (D:\RMS\visualstudio\Projects\ClassLibrary5\ClassLibrary5\bin\Debug\ClassLibrary5.dll'
WITH PERMISSION_SET = EXTERNAL_ACCESS
Does this work:
ALTER ASSEMBLY ClassLibrary5
FROM 'D:\RMS\visualstudio\Projects\ClassLibrary5\ClassLibrary5\bin\Debug\ClassLibrary5.dll'
WITH PERMISSION_SET = EXTERNAL_ACCESS