calculating seniority banking

I have table Fact_an

banking seniority

I create a report and a new mausure AN. WHE A USER SELECT A YEAR AND A MONTH AND banking seniority, I need to calculate the count of the rows the last 12 of months banking seniority< banking seniority seelcted by the user

AN = 

VAR a = SELECTEDVALUE(Dim_DateFicheAgent[ID_DateFicheAgent])
VAR b =SELECTEDVALUE('Seniority banking'[banking seniority])
RETURN

    CALCULATE (
COUNTROWS(FILTER(Fact_AN;

     (Fact_AN[banking seniority]<=b && NOT ISBLANK (Fact_AN[banking seniority]))));
         DATESBETWEEN (
        Dim_DateFicheAgent[ID_DateFicheAgent];
        NEXTDAY ( SAMEPERIODLASTYEAR (LASTDATE ( Dim_DateFicheAgent[ID_DateFicheAgent] ) ));
        LASTDATE ( Dim_DateFicheAgent[ID_DateFicheAgent] )

))

it returns errors wrong values. I put an example of the report here https://drive.google.com/file/d/1Ja3NevOm6i80uuS6lKPpHIBYaNQ2jee2/view?usp=drivesdk