Form 8606 has nothing to do with a distribution from a 401(a). Form 8606 only applies to conversions from traditional IRAs.
Your options were described your other thread:
https://ttlc.intuit.com/community/retirement/discussion/conversion-of-401a-to-roth-ira/01/1961043
Form 8606 would apply if you rolled the 401(a) distribution over to a traditional IRA, then you converted the traditional IRA to Roth. If that was the case, you would have a separate Form 1099-R from the traditional IRA to report.
