Make sure that there are no compile errors and that the file name and class name match - unity3d

Question

I keep getting the same error over and over no matter what I do.
"Can't add script component 'PlayerMove' because the script class
cannot be found. Make sure that there are no compile errors and that
the file name and class name match".
I am very sorry that this is very long, but this is the extent of what I have been trying to do to resolve this problem, so please be patient.
I have been working on this 2D project for three weeks. It was working fine until 3 days ago. So far, I had only created the environment and added characters who were doing nothing. There is music and effects. All of these is (still) working fine.
Then I added a script to make my character move. Here is the script:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class PlayerMove : MonoBehaviour {
public float moveSpeed;
public Rigidbody rb;
// Start is called before the first frame update
void Start()
{
rb = GetComponent<Rigidbody>();
}
// Update is called once per frame
void Update()
{
rb.velocity = new Vector3(Input.GetAxis("Horizontal") * moveSpeed, rb.velocity.y, rb.velocity.z);
}
}
I was so happy, my character was moving!! There were no errors! It was all fine. It worked like that for two more days after adding the script. I'm saying this to emphasize that Unity did not crash because of the script.
Unity crashed, this happened on Monday. Even though I did not lose my project files, and restarting Unity did not work, I had to uninstall and reinstall Unity. My version was 2019.4.1f1 before and after as well, been working on same version, since I don't want to get stuck with upgrade issues.
When I reinstalled Unity and reopened my project, unfortunately, Cinemachine and ProBuilder were both gone. This is another problem I have been facing since the start. But that should be another topic.
PART 1
After a lot of work, I gave up and decided to create a new project. Following are the steps I took:
I created a new project
installed CM, PB,
went and imported my project.
The script was not working so I reimported it.
PART 2
It still did not work, so I created a new script- with a different name- by using the method of right clicking in Assets and creating a script.
Then I just copied my THREE LINES OF CODE - literally as you can see above - and tried to add it to my character. Still not adding. It wants to know if the class name and file name are same - which they are of course - no matter how many files I create, in whatever method, I make sure the class name and file name are same. And the class does inherit from Monobehavior. Plz refer to error message in start.
PART 3
I then created another script file. I deleted the previous script, made sure there were no other scripts with similar names, then went to the PLAYER and created a file from the Add Component menu in the Inspector. I double clicked the file from there and then added the code. And after I had saved it in VS 2019, it's an invalid file in the Inspector in Unity.
I have tried doing this with several file name changes. And I have tried to import the script file from another folder. I keep getting the same error over and over again. Just now I finished reimporting all process as well. I even uninstalled VS 2017, and installed all latest .Net SDK Frameworks.
I'll be grateful to anyone who can help.
I also added this question to the Unity forums a week ago (I was getting same error message), and was successful in resolving it, but now that solution is not working.
Remember that I have created absolutely no other scripts. The only scripts present in my project would be the ones attached already to the assets, and there is no asset in my project that requires a movement from input, or can be called a "player character" game object.
Oh, and I also tried to re-drag (?) the Prefab of my Player asset from the main folder over to the scene view. I even just now reimported the whole thing - Reimport all.

Answer 1

I have been trying to shorten the question over and over, but the steps I took to resolve this issue were so many that I could not manage it.
However, good news is that I did finally resolve the issue.
Of all the things that could be expected, this is what worked: I went and deleted all "SmoothMoves" folders from my assets. I have no idea why that helped, I don't know the connection.
Please read this question I posted on Unity Answers - anyone with similar issues - as proof that I don't know what effect SmoothMoves files and folders had on my project and I don't know what effect removing it will have. (Yes, I know some of my character assets (enemies) use SmoothMoves, but I'm still not sure what was in those folders I deleted).
But my main player characters are both now moving. My script file added to the game object without so much as a blink.
https://answers.unity.com/questions/1753426/smoothmoves-dll-file-problem-can-anyone-please-hel.html
So this problem, make sure the class name and file name are same, an odd issue, can arise from SmoothMoves folders (I still have all character prefabs in the project, I only deleted the specific SM folders ONLY). Be careful.