Redeem multisig tx using bcoin with multiple inputs (from an exhcnage)
That guide might be a bit outdated. There is a newer guide that just runs through the CLI commands here.
You can also join us on bcoin slack or on IRC in the #bcoin channel.
We'd need more information from you about how you constructed your transaction. The error you're getting indicates that you are trying to spend from a non-existent output. It may never have existed, or it may have already been spent in the past.