The details are all in your error message. It will tell you what was being published: Usually a particular tag within a specific template. That’s the template you need to go to, and in that template, you need to check for the missing value, to determine what you need to do next.
The missing values are usually used in default templates as attributes supplied to the mt:Link template tag. By using these special values, the mt:Link tag can look up the templates by name, rather than including a URL by hardcoding it. That way, if you change the URL in a template, it flows through to the template that includes the URL. In other words, change it in one place and it flows through to all the other places that use it. But there are two potential problems with this approach.
The other option is if you have actually removed the template in question, such as a stylesheet. Perhaps you are hosting it elsewhere, maybe on a Content Delivery Network (CDN) or it is being maintained outside of Movable Type. In this case, you may not have the template inside of Movable Type. In this case, you can still keep the template in Movable Type, and use the template type field, and just set the publishing option to do not publish, or you can go into the original template (the one causing errors) and enter the value manually, rather than using the mt:Link template tag. Then save it and publish again.