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Divya ManukondaDivya Manukonda 

The 'Opportunities by Rep and Close Month' summary format is not 'Matrix'.

i got the error i tried all the solutions posted in answer session tried sorting closed date group by column 
tried deleting avaliable reports on the same name

tried everything help me solve my case in GROUPS AND CATERGORIZE YOUR DATA
Sai PraveenSai Praveen (Salesforce Developers) 
HI Divya,

Please note that Questions about how to pass Trailhead challenges are not on topic, because these challenges are intended to be independent demonstrations of your abilities.

Trailhead Help (https://trailhead.salesforce.com/en/help?support=home)

can provide assistance for situations where Trailhead does not appear to be functioning correctly. You can reach out to them if this is the case.Hope above information helps.

Please mark as Best Answer so that it can help others in future.