First, you don'r need a for loop that runs over system, sine you re just assigning it one value! That is, this:
is silly and unecessary. Just write
(removing the now spurious end that went with the for, of course.)
Next, the if is written incorrectly, as MATLAB told you. Just because it is common mathematical shorthand to write this:
(0.8 < B < 1.2)
does not make it work as you wish in MATLAB. Split it into two tests, which is what the shorthand implies anyway.
if ((A == 45) && (0.8 < B) && (B < 1.2))
Finally, what was the immediate reason why your code failed in the first place?
'Operands to the || and && operators must be convertible to logical scalar values.'
That tells you either A or B, or both of them, were not scalar variables. You cannot have an if statement that loops over a vector or array. That is, an if statement is not an implied loop.
I would also STRONGLY recommend that you read the getting started tutorials. If you have made 3 basic mistakes in your first 3 lines of code, this tells me you never bothered to read any documentation. READ THE MANUAL. It was put there to get you started.