dx.jar was not loaded from the SDK folder

Question

I have an old project, which was made in Eclipse Luna.

Now when I open it in this version of Eclipse or in Eclipse Photon, then such an error occurs "Unable to build: the file dx.jar was not loaded from the SDK folder!".

This error occurs when the project is compiled.

I know that there are already some questions/answers about it in this forum but they didn't work for me.

The methods I tried:

1. Reset Android SDK
2. Reset Eclipse
3. Clean project
4. Android ADT error, dx.jar was not loaded from the SDK folder
5. https://www.buildbox.com/forum/index.php?threads/how-to-solve-it-the-file-dx-jar-was-not-loaded-from-the-sdk-folder.10509/
6. http://davidcrowley.me/?p=663
7. https://code.i-harness.com/en/q/aa933f
8. Unable to build: the file dx.jar was not loaded from the SDK folder in my eclipse
9. Unknown error: Unable to build: the file dx.jar was not loaded from the SDK folder