A quick google told me this isn't possible, but was from a while ago. Can I create a workflow trigger based on a Line item field on a Bill? I want to return a user error if Amortization Start and End date is filled out, but a template is not selected.

No - but I imagine you can set up a saved search monitoring this instead, so that it can be fixed, right?

That is exactly what I did actually! Originally we just had one showing the results, but I just set it up to send email alerts when it happens.