If Republicans have Project 2025. Why don't Democrats have their own guide book?

We do. It’s called the Constitution of the United States of America.
Clearly, Republicans don’t care for it much.

I couldn't tell you specifically why, but I would question the value of such a project. Project 2025 undermines the foundations of the United States and moves the US towards authoritarianism.
Red versus Blue was an entertaining web series, but not an effective system of governance. Things like project 2025 are symptoms of a deeper sickness in the United States. Democrats shouldn't be playing the same game as fascists, once you play by their rules, you've already lost.
The United States is an Oligarchy, the will of the people is almost entirely eclipsed by the interests of capital. Democrats don't need their own project 2025, the people need representation in government regardless of what color flag their elected officials fly.

They do, it just uses the generic name "platform" to refer to their political strategy instead of naming it something ominous.

As well as the other good answers you've received here is also the fact that they are in power currently. If dems were out of power outside groups would be publishing policy books and papers to influence the direction going forward in the coming retool. They probably are now. But as has been said the dems have an actual platform to attack so only policy wonks and fever swamp dwellers read and attack the papers coming out of democratic leaning institutions right now.

ours is common sense really
- Liberals helped get us into this corporate hell hole. Don't let either party off the hook. That being said. The stakes this election are too high.
  Vote blue.

Because when the Republicans are in power they fall in line, when Democrats are in power they fall in love.
To explain that last bit, Democrat candidates tend to have more direct goals, more local or state policies or projects they would like to enact. So everyone of them arrives in DC with something they are trying to do, and nothing usually happens as it requires the party to agree on it. Gathering the will to collectivly push past the GOP is usually too high of a barrier for most modest bills, especially when they are being extra obstructionist.

Even if they did, you think they'd get it released? They saw what happened to the Heritage Foundation following the leak of P'25.
- if it leaks and it’s a problem, it probably isn’t a good idea to go forward with that policy.
  Which, for the record, democrats aren’t likely to have a “secret manifesto of things only assholes want” for their policy guidebook. (But they do have a policy guidebook. They develop one every National Convention )

Logic and reason seems to have worked faulty well so far. There’s room for improvement though.

- They used to rant and rave about some progressive manifesto they were worried about. It was around the time that Hilary in the primaries the first time.
  I've never heard about that. Please cite a reputable source so I can read more about it.
  
  I can't find the paticular document so that's why I had already deleted my post.
  I remember it was some document that my conservative acquaintances were all worried about. When Project 2025 came to light the liberal reaction to that reminded me of the reaction at the time to Hillary's progressive plan. Really this would have been early 2000s and a "radical progressive" agenda would probably be considered mild by most today.

That would require having policies, and not just a campaign strategy of "We're not as bad as the Other Guy!"
- They literally have a 78 page document on their economic plan, idk what you want