Cannot add button object to Script in Inspector

For some reason, in my project, I am unable to add a simple Button UI to a public Button, inside the inspector. I am able to if I create a new project.

Simple Test in project that does not work(New scenes do not work either)

1. Create a button(if new scene it creates the canvas parent, as expected)

2. Create empty Object

3. Add script to object with the following code only

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class ButtonTesting : MonoBehaviour {
 
     public Button buttonTest;
 }
 

When I try to drag the button to the Button Test script in the inspector, it highlights, but does not stick. If I click the little circle to the right of the Button Test in the inspector, the Assets/Scene window pops up, but nothing is listed.

In my other projects, or if I start a new project, this works perfectly as expected. I have no idea why it is not working in my one project, and I would prefer not to have to start over of course. I have never encountered this issue before, and I have restarted Unity multiple times to try to fix it.