I was having the same issue on Windows when I had previously created different projects on the undefined workspace (the default workdspace of VSCode).

When I create a workspace and I placed my root folders inside this one workspace, it will warn that it could not resolve workspace folder.

You have to edit your workspace config file, change the path of your folders and then restart VSCode.

On the VSCode command palette, type: workspace config - then choose "Open workspace configuration file." You should have something like this:

{
"folders": [
    {
        "path": "OneProject"
    },
    {
        "path": "AnotherProject"
    }
],
"settings": {}

}

What you want is something like this:

{
"folders": [
    {
        "path": "C:/Somewhere/OneProject"
    },
    {
        "path": "C:/Somewhere/AnotherProject"
    }
],
"settings": {}

}

It's a known issue, it was fixed here - https://github.com/Microsoft/vscode/issues/50866

As of the 1.24.1 version it's not yet available. But the fix listed above should correct the problem you are having.