Harsh
2
Hello @pedroccamara,
Thank You for posting your query onto the Forum.
You can try the formula given below -
Sales LY =
SUMX(
SUMMARIZE( Dates , Dates[Year & Month] , "Last Year Sales" , [Sales LY] ) ,
"Last Year Sales" )
I’m also providing a link below where one of our expert @Greg had created a post which addresses specifically this issue.
Hoping this helps you. 
Thanks & Warm Regards,
Harsh
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