Four quick troubleshooting tips:

If you get a window that looks like this, there is a good chance that Apple still supports your Mac. Double check to make sure of this.

Try giving OpenCore special permissions: go to System Preferences > Security > Full Disk Access > and allow OpenCore to have full disk access.

You may have to upgrade macOS before you can install OpenCore on the Hard Drive itself. In this case, install OpenCore (using the same settings) on an external USB drive.