I had a similar problem when first upgrading to Julia 1.11. The issue was the Manfest.toml file from an older Julia version causing 1.11 to error. If that is also causing your issue, the solution is either deleting the Manifest.toml file in the current directory or editing your kernel to not load the manifest. In other words, delete the --project=@. option from the kernel. See this for more details: Jupyter cannot find the Julia 1.11 kernel - #9 by kpobrien