"Unable to load map specified by server" (just trying to edit normal DotA, nothing fancy)

Hello! I dabbled in modding ~2 years ago, implementing some custom heroes into otherwise normal DotA. I wanted to pick it back up again to implement a Techies rework I had in mind. However, I have somehow managed to hit a wall from the start: I can't even get the map to load haha

Essentially, when I do the following steps that I seem to remember from two years ago:

1) Open the addon

2) Open Hammer

3) Open the map to be built from the addon 'map' files, which in this case is just the normal DotA map

4) Build the map

5) Run the map

I get stonewalled by the message: "Unable to load map specified by server". Apparently either things have changed or I have forgotten the necessary steps.

So, two questions:

A) What can I do to fix this specific issue? I just want to make changes to a single hero and leave DotA otherwise the same.

B) More broadly, is there a guide anywhere on how to implement normal DotA with minor changes? Every tutorial I can find is geared towards making your own map in Hammer, making a fresh addon, etc. I've already done it once and remember it being a struggle back then, but again, apparently either things have changed or I have forgotten them.

Thank you for your time and feedback!