If you see the Apple Logo, and then the progress bar, you are in good hands.

But, if you see the Prohibitory Sign, you will need to Spoof your Mac's Serial ID. This is extremely rare, and run back to Guide #3 to make sure you installed OpenCore correctly.

Once again, I will soon make a guide covering OpenCore's Post-Installation. That will cover Spoofing, Feature Unlock, Customizing the OCLP Picker, etc.