Is it correct to say it is paid on every month or in every month?

Answer 1

Either of those is a possibility. if you have a savings account that accrues interest, you may get a deposit "in every month" because one shows up in January, one in February, and so forth. But you also are getting "on every month" since the bank is paying you on the basis that the money was in the account for a particular 30-day period.

Answer 2

It is correct to say "it is paid every month." This indicates that the payment occurs on a monthly basis. Using "in" instead of "every" would also be acceptable, as in "it is paid in every month," but it may sound slightly less natural in this context.