Can't create Collection using ClearCollect Function

[RezaDorrani]

@carem 

Ok so it means something is incorrect in your data

CountRows(Filter( Schools, Faculty.Value ="hardcoded value" ))

When you go to SP list Schools - does it have any data? I am assuming Yes

Does it have values in Faculty column - see the values there

then try and hardcode that value exactly in the hardcoded value in formula above and test

[carem]

@RezaDorrani 

I have discovered a serious problem with Preview Mode. I think it is the cause of this problem.

My Preview Function is not working. It's mean when I press F5 you can see my form like this:

Edit ModePreview Mode

It may be that it cannot recognize the value of drpd_Faculty.Selected.Value. And return an empty Collection.
Follow Pstork1's advice, I create a button with OnSelect Property:

ClearCollect( collSchools, { Schools: Filter( Schools, Faculty.Value = "Faculty of Health" ) } )

And it's work.

But when I go back for  

ClearCollect( collSchools, { Schools: Filter( Schools, Faculty.Value = drpd_Faculty.Selected.Value ) } )

It does not work anymore.