Load order had mods that were missing

[Noggog]

Synthesis has a blocking error that says Required Mods Missing From LoadOrder with some mods listed.

[Noggog]

Cause

Synthesis is being run in an environment where it does not see a mod(s) that one of the patchers you have installed requires.

Fixes

Install the mod

Simplest issue might be that you just don't have that mod in question installed. Go get the mod the patcher requires and install it.

Ensure Synthesis is being run through your mod manager

Mod managers like Mo2 are in charge of running applications and "tricking" them to think the mods are inside your skyrim Data folder. If you run applications like Synthesis straight from the desktop, then Synthesis might see a mostly empty Data folder. By running Synthesis through Mo2, it will then magically see all the mods within the data folder and be able to see/use them as intended.

Ensure Missing Mod is Before Patch in Load Order

Synthesis ignores mods that come after its target patch mod in the load order. So the mod a patcher requires might exist and be active, but if it comes after the patch mod in the load order, then from Synthesis' point of view it does not exist. Put the mod before the patch that is going to use it.

Multiple installations might cause problems

If you have multiple installations of the same game, this can sometimes cause problems. Synthesis is run through Mo2, but still complains that it cannot see any of the mods you have installed. This could be cause by Mo2 applying its mods to Installation A, while Synthesis is looking in the registry and finding/inspecting Installation B. So it legitimately cannot see any mods, but this is because it is looking in the wrong location.

Synthesis looks to the default installation location, as described by registries or Steam. If you want it to look elsewhere for the data folder, you'll need to modify that in the profile settings.
https://github.com/Mutagen-Modding/Synthesis/wiki/Overriding-Data-Folder-Path

Remove the duplicate installations

Usually multiple installations of the same game is unnecessary. Mo2 and other mod managers are tools that allow you to have one single installation, but apply different sets of mods to that single installation at will. As such, it's usually simpler just to have one installation on a computer.

Set Synthesis Data Folder Settings

If having multiple installations is necessary, then make sure to set Synthesis' Data Folder path manually.

Other Problems

Some users have reported still having issues after considering all the potential causes above. So far 99%+ of them have ended up being user error of some sort. Tinkering with and learning their Mo2 settings eventually results in them fixing the issue and starting Synthesis in an environment where it is able to see the mods.

If anyone has any pointers or typical pitfalls that new users fall into on this, feel free to post them so others might know what to look for!

[Sumerianhaze]

No donor mods could be found issue or No mods in expected data folder issue.

Moved my MO2 installation from C to E drive & existing install of Synthesis could no longer find my donor mods. With no clear way to resolve this and in the process of trying to sort the problem I downloaded a new version of synthesis (btw silly name, try doing a Google search to actually find anything!)
New install had similar but different issue stating that whilst I had loads of mods it couldn't find them? They weren't in my data folder, which Synthesis pre filled, I corrected the location but still no joy.

Resolution was to delete location of data folder location in profile and just leave the field blank. see the bottom of the picture above.

[sachinkmr]

Updating the Stock Game folder path for the wabbajack-based modlists in registry editor worked for me.

Ex:
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Bethesda Softworks\Skyrim Special Edition]
"Installed Path"="E:\\Modded\\SSE\\Skybax\\"