Fill line chart with last know value - using date table

The red line under PRICE isn't really an error, its just Intellisense playing up, it does that sometimes.

The code is intended for use as a measure, but the SUMX is performing the aggregation, you don't need another aggregation function inside it.

I think the problem is the relationship with the date table, try

Last Known Price =
VAR RefDate =
    MAX ( DF_Dim_Time[Date] )
VAR Result =
    CALCULATE (
        SUMX (
            INDEX (
                1,
                FILTER (
                    tbl_price,
                    tbl_price[PRICE_UNTIL] >= RefDate
                        && tbl_price[PRICE_FROM] <= RefDate
                ),
                ORDERBY (
                    tbl_price[PRICE_UNTIL], DESC,
                    tbl_price[PRICE_FROM], DESC,
                    tbl_price[Index]
                ),
                PARTITIONBY ( tbl_price[ARTPREINH] )
            ),
            tbl_price[PRICE]
        ),
        REMOVEFILTERS ( DF_Dim_Time )
    )
RETURN
    Result

or you could remove the relationship entirely if you don't need it for anything else. What the relationship is doing is filtering only those records with a price from ( or to ) date which exactly matches what is in the date table, and that isn't really useful for records which span multiple dates.