If I've entered an amount in Box 12 that is for Code M, why is it asking me to enter an amount that is attributable to RRTA Tier 2?

It's a required question.  Box M means that you are a former employee and received more that 50,000 of life insurance coverage, which is more than what the IRS will allow your former employer to cover pre-tax or untaxed.  Any premiums for coverage amounts above 50,000 must be included in taxable income and subject to either Social Security/Medicare payment or Railroad Retirement payments.  If your job is not associated with the Railroad, you may leave that box blank.  

Here's an FAQ that describes the box 12 entries, to assist you to understand the box 12 code you see:  https://ttlc.intuit.com/replies/3300234