Looks like you are in dependency hell;)
Pkg.add(name="Zygote", version="0.6.17")should tell you, which package is preventing the update.
You are correct. On one of my devices, this worked. On the other, it told me which package was preventing it from updating (ChainRules), however I was not able to delete that package. I think it was because my manifest.toml was messed up. So I just deleted the entire .julia folder, reinstalled all the packages with Pkg.add (or ]add ), and it seems to be working now. Thank you!
For someone investigating the original bug, seems to be an error in Zygote that was fixed at some point. Update Zygote.