If you are trying to sum the column Table 1::B and Table 1::C, the formula is
=SUM(Table 1::B, Table 1::C)
It should have a comma, not a +
A "whole column" reference like Table 1::B can mean the entire column or it could mean "the cell in Table 1::B in the same row as this formula". If Table 1::B is used in a formula in a spot that requires a single cell, it will be treated as a single cell. If it could be a range or a single cell, it is interpreted as a range. Same goes for the reference to column C. In your formula you have Table 1::B + C. You cannot use + on a range so both of those references are single cells. Your formula is in row 1 so it is the same as Table 1::B1 + Table 1::C1
Same for the formula in row 2. SUM(E+F) is the same as SUM(E2+F2). You want SUM(E,F)
It is a lot easier to provide help when "use header names as references" is unselected. That "C" reference threw me off for a while. I thought it was column C in Table 2 (which does not exist) but it is actually the column in Table 1 that has a "C" in its header row (which happens to be column C in that table)