Hello!
bm_backa is a variable that must belong to some selection. You can't refer to it without specifying that selection. So it should be, for example, football7_s_1_bm_backa to address the amount of the latest back matched bet of "Under 1.5 Goals", because it is the first in the list of the two selections. Or it can be football7_r_1_bm_backa to address the amount of the back bet placed on the lowest priced selection in that market.
You can't just use football7_bm_backa, if you see what I mean.