That is your problem. That is a True/False. You are essentialy checking if A = True or False. Let's Assume A = 5 and B = 10 and break that down.
TagA = TagB +1
This resolves to if 5 = 11 which is a valid statement but would be false with the current values.
OR
if TagA = TagA = 1 (5 = 5 = 1)
This of course does not make logical sense. For example, change that to A + 1 and you have a valid statement that the editor will accept.
Someone recently claimed a logical-OR was added to the Logix 5000 comparison expression operators, but as far as anyone replied it was not in version 30 or less if I remember correctly. Not sure if anyone checked the newer versions.