Hi Mukul,
Your formula is trying to compare 2 dimensions data type metric - Month, and as the error indicate, the only syntax allowed for dimensions comparison is “=” or “<>”, not “>” or “<”
The hint here is use the start date/end date of the month and then run the comparison. To get History period boolean in this case, I suggest you use this formula:
IF(Month.'Start Date' < 'Current Day', TRUE, BLANK)
Why BLANK and not FALSE? Looking at your formula, the other problem I forsee here is you use FALSE for the second arguments of your IF. As for this boolean, you only care about the Months that are history, then you don’t really care about the rest of other months. For IF argument you don’t really need, you should put BLANK instead of FALSE, why? As although the results are still correct but FALSE will densify your metric and effect overall related performance unncessarily.
Hope my answer helps