....click on the expression button next to the parameter value and look at the expression within. If it's in the incorrect case e.g. =Parameters!eDate.Value you need to correct it...

Table with sample Data:

DROP TABLE #OE_LINE_RICH
CREATE TABLE #OE_LINE_RICH(
 order_no       VARCHAR(8) NOT NULL PRIMARY KEY
,line_no       DECIMAL(19, 0) NOT NULL

sorry for the lack of detail, here's what I tried:
   BEGIN
    UPDATE o
    SET generic_custom_description = i.order_no +' '+ RTrim(LTrim(SUBSTRING(i.extended_desc, CharIndex('Customer Bin:',i.extended_desc)+13,255)))
    FROM oe_line o
    INNER JOIN INSERTED...