Sebastian,
G2:U2 is an address range. It is not a Function, so Numbers doesn't know what you want to do with that range of cells. An address range can be, for instance, the parameter for a number of Functions that deal with a collection of values. SUM is one Function that operates on a collection of values.
Just for fun, drag across a range of cells, in a row, or column, or a rectangular selection, and look at the Statistics for that collection displayed at bottom of the Window. See below an example where I selected all the cells in the "Actual" column of a table.

Now, if you wish, you can Drag one of those capsulized statistics calculations into a table cell and the result will be the same as if you had programmed that cell that you dragged the capsule into. A great way to explore some functions.
Jerry