A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
That error is returned when you have linked data types. When you use =sum(b2.b5) the dot means that your formula is looking for the data b5 of field b2, neither of which exist. Use
=SUM(B2:B5)
Note the colon, not period, and all should be well.
(Note: November 27, 2018. Data types have been released to all Windows Office 365 subscribers, except in a few geopolitical locations (coming soon). If you are an Office 365 subscriber, make sure you have the latest version of Office.)