Selective replace mcode

the syntax you are using is wrong. Suppose you have a query called your_table that brings you this initial table: 

 Create blank query, open advanced editor and replace everything inside it with the following code: 

Table.ReplaceMatchingRows(
    your_table, 
    {
        {[Staff Number = 21000361, Warehouse = "Unknown"], [Staff Number = 21000361, Warehouse = "Gate 2"]},
        {[Staff Number = 21007164, Warehouse = "Unknown"], [Staff Number = 21007164, Warehouse = "Gate 5"]}
    }
)

Result is: 

Make sure you know what are lists and records as well as list of records and their syntax in M. Otherwise it won't be easy for you to implement and support my solution.