Hi @Racheli_Zamari ,
Please look at the following eazyBI community post - Calculate Cumulative Sum only for the rows of a Particular Issue. It is pretty similar to your request.
You can see the suggested solution with a calculated measure, considering the Transition dimension and the “Workdays in transition status” measure. You can re-use the calculated measure formula to calculate the cumulative Sum by Epic per Sprint. Replace the Transition dimension with the Epic Link dimension and the measure with “Sprint Story Points completed”.
Let me know if you get stuck or have any questions.
Best,
Roberts // support@eazybi.com