what does "no mode specified" error means in suiteCRM

hi, i’am getting this error all the time, what does it mean how to resolve it.

Windows server 2012 r2

No issues from me using Windows server 2012 r2 with IIS
Could it be that your server lacks some modules/features installed?

On the “no mode specified” it can be related to Handlers.
Are you using the integrated or classic Mode for the application?
I think I can’t help you on that!

i don’t know where the problem come from, i tried changing permissions to full control. and still the same error, it’s two weeks now with the same problem.

i’am using the integrated AppPool

what are Handlers? could you clarify please