How can I enter the old passcode after I have reset the phone because I could not remember the old passcode!?

I may be wrong, but I interpret your question a little differently.

Are you trying to enter your old passcode to use on the phone after you erased it because your could not remember the passcode? If you did erase the phone and you are starting over again, you can use the old passcode if that’s what you want to do.

This support article explains how to enter or change your passcode.

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

I apologize if I am misunderstanding what you are trying to do.

Sorry, too late to recover the data that was erased, if that is what you are really asking here. If not, please clarify.