Please follow the below steps, these worked well for me in Eclipse.

To Resolve "TestNG Class Not Found" Issue in Eclipse:

1. Right-click on your project in the Project Explorer.

2. Select Build Path → Configure Build Path.

3. In the Libraries tab:

   • Ensure the TestNG library is listed and checked.

   • If it’s missing, click Add Library → TestNG → Next → Finish.

4. Switch to the Order and Export tab:

   • Check the box next to the TestNG library.

   • Move TestNG to the top of the list.

5. Also ensure the JRE System Library is added and points to a valid JDK (Java Development Kit), not just the JRE.

   • You don't need to move the JDK to the top, just make sure it’s checked.

6. Click Apply, then OK to save the changes.

7. Right-click the project again and select Maven → Update Project.

8. Run a Maven install to ensure dependencies are resolved:
   Run As → Maven install

9. Finally, run your class as a TestNG Test:
   Right-click → Run As → TestNG Test

Happy Coding!!
Cheers!