Need help to build Simple Script

For the only one layer in document unless you mean also layers in layer set.

!(activeDocument.layers.length - 1)

For checking existence of following file on desktop:

File('~/desktop/someFile.psd').exists

ExtendScript Debugger

Your requirements are not crystal clear.

1) There is a conditional action to check for whether an image has layers (1 or more) or is flattened with a special Background layer, which technically is a single layer, but this may not meet your criteria?

2) Conditional actions don't include paths, so you would need a script for this one anyway.

It is easy enough to script a warning/alert when opening a document (Script Events Manager ) if it contains > 1 layer or path, however, if you want automation to actually "do something" then more info would be required.

If you are using Apple ScriptEditor, then don't use it for Adobe JavaScript, more here on editors:

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html

if (activeDocument.layers.length > 1) {
    alert("There is more than 1 layer!");
}

Note: An empty layer set/group counts as a layer with the simple code above.

if (activeDocument.pathItems.length > 1) {
    alert("There is more than 1 path!");
}