Hey! Sorry you're running into this. A few things worth checking before we dig deeper:

1. Check the LP's profile - make sure they have an email address on file. A blank "To" field is a known cause of this exact error.
2. Check your PACT email template - the Send PACT button looks for a template named exactly "5.1. PACT: Send (v1)" or "5.1. PACT: Send". If that template was renamed or deleted, it can cause send failures.
3. Try the pipeline method instead - go to Fundraising, find your prospect, click "Send Email," and select the PACT template from the dropdown. This tends to be more reliable than the blue + button.

If none of those do the trick, we'll look into it on our end! Could you share a screenshot of the emailer right before you hit send? That helps us track down the issue in the logs.