Why can I not access the `totals` box?

[#1]

dept	date	sum(orders)	Sum (staff wages)	sum(staff overtime)

I have a table that looks like this.  the values are displayed but for some reason I cannot choose the options under 'total mode' (expressions tab).  do I have to change a setting or is there possibly something wrong with how I inputted the data?

also I would like to add a column somehow that adds `staff wages and staff overtime` to get a staff total cost, what expression do I use?

would really appreciate any help on the topic.

thank you in advance