Slack slash commands trigger not authorized

I activated Slack slash commands for a project but the trigger endpoint only returns:

{“response_type”:“ephemeral”,“status”:200,“text”:":sweat_smile: Couldn’t identify you, nor can I authorize you!"}

I tried adding a personal auth token to the request but that didn’t help.

I’m not actually using Slack but trying to integrate with Zulip using their Slack-compatible format, if that matters.

Help please.