You are trying to use the value of the $fuerzatropas1 story variable in a mathematical expression ...
<<set $fuerzatropas = $fuerzatropas1 / $variable1>>
... before you assigned it a numerical value in the next line. The same is true for your $tropas story variable which also hasn't been assigned a numerical value.
By default all SugarCube story versions story variables start as undefined and using an undefined story variable within a mathematical expression will result in a Not-a-Number error, because the value of the undefined variable isn't a number.
To solve your issue you need to first assign an initial value to the $tropas variable and then change the order of the last two lines in your example...
<<set $fuerzatropas1 = $variable02 + $variable03 + $tropas>>
<<set $fuerzatropas = $fuerzatropas1 / $variable1>>
... so the value of $fuerzatropas1 is calculate before you reference it in the other mathematical expression.