find the difference date between two unique car number

You will need a column which uniquely identifies each row. If you don't have one already you can use Power Query to add an index column. In the modelling view you need to select the table and mark the unique column as the key column for the table.

Then you could create a measure like

Avg days between repair =
AVERAGEX (
    'Table',
    VAR CurrentRow =
        ROWNUMBER (
            ALL ( 'Table' ),
            ORDERBY ( 'Table'[Repair date], ASC ),
            PARTITIONBY ( 'Table'[Car number] )
        )
    RETURN
        IF (
            CurrentRow > 1,
            VAR CurrentDate = 'Table'[Repair date]
            VAR PrevDate =
                SELECTCOLUMNS (
                    OFFSET (
                        -1,
                        ALL ( 'Table' ),
                        ORDERBY ( 'Table'[Repair date], ASC ),
                        PARTITIONBY ( 'Table'[Car number] )
                    ),
                    "@repair date", 'Table'[Repair date]
                )
            RETURN
                DATEDIFF ( PrevDate, CurrentDate, DAY )
        )
)