period_end has to be a field, not an expression and not a calculated dimension.
Make sure that the field period_end contains date values in the format YYYY-MM-DD hh:mm:ss or text values.
Make sure the field string_value contains numbers that can be summed. Summing text values will not work unless you use the num#() function to turn the strings into numbers.
Make sure to use the exact case-sensitive field names. Period_End and period_end are two different things to QV.