Measure not working with Slicers: DAX

Hi,garimajain85.Hello, danextian, nandic.Thank you very much for your help with this issue.
And now I want to share my advice with you.
I am glad to help you.

Regarding the issue you raised, my solution is as follows:
1.First on your first question, the reason why metrics are affected when using a slicer is because the slicer changes the context, which may cause some metrics to return an error in that context, here's a link to that:

Context in DAX Formulas - Microsoft Support

2.First I have created the following table and the column names and data are the data you have given:

3. Below are the measure I've created for your needs:

Measure 2 = SWITCH(TRUE(),
ISFILTERED('M'[manager]),IF(MONTH(SELECTEDVALUE(MON[1]))=MONTH(TODAY())-1,
DIVIDE(CALCULATE(SUM('IFERROR'[Active count of employees]),FILTER(ALLSELECTED('IFERROR'),'IFERROR'[manager]=MAX('IFERROR'[manager]))),AVERAGE('IFERROR'[Active count of employees])),
FALSE()),
SELECTEDVALUE(MON[1])<=TODAY(),DIVIDE(CALCULATE(SUM('IFERROR'[SALE]),FILTER(ALLSELECTED('IFERROR'),MONTH('IFERROR'[month])=MONTH(SELECTEDVALUE(MON[1])))),
CALCULATE(SUM('IFERROR'[Active count of employees]),FILTER(ALLSELECTED('IFERROR'),MONTH('IFERROR'[month])=MONTH(SELECTEDVALUE(MON[1]))))),
SELECTEDVALUE(MON[1])>TODAY(),DIVIDE(SUM('IFERROR'[SALE]),
CALCULATE(MAX('IFERROR'[Active count of employees]),FILTER(ALLSELECTED('IFERROR'),MONTH('IFERROR'[month])<MONTH(SELECTEDVALUE(MON[1]))))),
FALSE())

Measure = 
VAR _a = SELECTEDVALUE('MON'[1])
RETURN IF(_a =BLANK(),DIVIDE(SUM('IFERROR'[SALE]),SUM('IFERROR'[Active count of employees])), 
[Measure 2])

4.Here's my final result, which I hope meets your requirements.

Can you share sample data and sample output in tabular format if I am misunderstanding? Or a sample pbix after removing sensitive data. We can better understand the problem and help you.
Best Regards,
Leroy Lu
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