Typically in this situation I see people confusing the profile/installation list for the version list.
Go to the Installations tab and create a new one, you should be able to select Forge from the version dropdown in there.
I've split this off into its own thread.
Please show a screenshot of the versions list in the launcher.
(Please note that for whatever reason, 1.12.2 Forge gets booted to the bottom of the list)
Forge installs itself as a version of Minecraft.
Make a new profile/installation and select the Forge version from the version drop down.
Also, don’t hijack threads.
There's no SetupDecompWorkspace anymore.
If you're using Eclipse, run the eclipse task.
If you're using intellij, just import the project and intellij will handle it.
The docs need to be updated, going from 1.12 to 1.14 is not as simple as changing the Forge version because ForgeGradle was redesigned.
You'd be better off getting a fresh MDK for 1.14.
If you don't know Java and are just going through the motions of whatever you are told you are not going to have a good time, so it's a valid question.
Unless you specifically tell Windows not to use OneDrive while installing Windows your user folders (Documents, Music, Pictures, Desktop, Videos, etc.) are put in OneDrive by default.
Please provide your installer log, it will have the same name as the installer but with .log on the end:
Installer: forge-{version}-installer.jar
Log: forge-{version}-installer.jar.log
Note: If you have Windows Explorer set to hide file extensions(as is the default setting), they will look like this:
Installer: forge-{version}-installer
Log: forge-{version}-installer.jar
I am telling you to toss out the mods you got form 9Minecraft and get the mods from CurseForge, try to reproduce the issue, and if it happens send the new log.