If you encounter the error 'Could not copy obj debug netcoreapp2 to bin debug' while working on a .NET Core application, it can be frustrating. This issue can occur due to various reasons, such as file permission problems, conflict with another process, or a corrupt project file. To troubleshoot this issue, you can try the following steps:

1. Check file permissions: Ensure that the files in the obj debug netcoreapp2 directory are not set to read-only. You can also try running Visual Studio or your IDE with administrative privileges to see if that resolves the issue.

2. Close conflicting processes: Sometimes, another process might be holding a lock on the files in the obj debug netcoreapp2 directory, preventing them from being copied to the bin debug directory. Close any unnecessary processes and try building your project again.

3. Clean and rebuild your project: Sometimes, corrupt project files can cause this issue. Try cleaning your project and then rebuilding it to see if that resolves the problem.

4. Check for file path length: In some cases, the file paths in your project might be too long, causing issues with copying files. Try shortening the file paths or restructuring your project directory to see if that helps.

5. Update your tooling: Make sure that you are using the latest version of .NET Core and any relevant tooling. Sometimes, issues like this can be fixed by updating your development environment.

By following these steps, you should be able to troubleshoot and resolve the issue of not being able to copy obj debug netcoreapp2 to bin debug in your .NET Core application. If you continue to encounter this problem, consider seeking help from the community or filing a bug report with the relevant tools or frameworks you are using.