Command not found in VSCode extension

You need to add all registered commands to the activationEvents list in package.json so that they are available on invocation. Update your package.json as such:

{
    ...
    "activationEvents": [
        "onCommand:extension.openMyExtension",
        "onCommand:extension.useMyExtension"
    ]
    ...
}

You can find more details on activation events in the official VSCode Documentation.