Hey man, hopefully you have gotten this fixed by now, but instead of running "setupDecompWorkspace" try "setupDevWorkspace"
it seems to have worked for me at first glance. No idea if this will mess me up in the long run with modding. /shrug if it doesn't work for me ill quit trying to mod lol.
I got this idea from reading the README that comes with Forge.
"Tip:
If you do not care about seeing Minecraft's source code you can replace "setupDecompWorkspace" with one of the following:
"setupDevWorkspace": Will patch, deobfusicated, and gather required assets to run minecraft, but will not generated human readable source code.
"setupCIWorkspace": Same as Dev but will not download any assets. This is useful in build servers as it is the fastest because it does the least work.
Tip:
When using Decomp workspace, the Minecraft source code is NOT added to your workspace in a editable way. Minecraft is treated like a normal Library. Sources are there for documentation and research purposes and usually can be accessed under the 'referenced libraries' section of your IDE.
Forge source installation"