Retention Rate

Gaurav_Lakhotia , refer

I think you need rolling 3 and 6

example

Rolling 3 = CALCULATE(sum(Sales[Sales Amount]),DATESINPERIOD('Date'[Date],ENDOFMONTH(Sales[Sales Date]),-3,MONTH))
Rolling 3 = CALCULATE(sum(Sales[Sales Amount]),DATESINPERIOD('Date'[Date],max(Sales[Sales Date]),-3,MONTH))

Rolling 6 = CALCULATE(sum(Sales[Sales Amount]),DATESINPERIOD('Date'[Date],ENDOFMONTH(Sales[Sales Date]),-6,MONTH))
Rolling 6 = CALCULATE(sum(Sales[Sales Amount]),DATESINPERIOD('Date'[Date],max(Sales[Sales Date]),-6,MONTH))

Will be blank for the customer that do not have data , so you can use +0 , so get all customers

Make sure date table is marked as date table

new vs repeat customer
https://community.powerbi.com/t5/Desktop/Churn-Rate-lost-Customer/m-p/1173754#M529196
https://blog.enterprisedna.co/new-vs-existing-customers-advanced-analytics-w-dax/
https://www.sqlbi.com/articles/computing-new-customers-in-dax/