Why do I have to enter a screen time passcode to erase my old iPhone when I don’t have such a passcode?

You may have forgotten the Screen Time Passcode.

If you have forgotten your Screen Time passcode

Use this link to reset your passcode If you forgot your Screen Time passcode - Apple Support

If you are unable to reset Screen Time passcode from the this-> link then, you will need a computer. If you don't have one, borrow from a friend.

Forgot? Click below to reset…

Click->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Regarding your data, If…

1. You have an iCloud or iTunes backup, I am sorry there is no use of a backup because if you restore from your backup your Screen Time Passcode will be back. 
2. You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive; like the information that are synced and stored in iCloud are, Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive. (*-If you had turned on iCloud Photos, your content is automatically stored in iCloud.)
3. You have imported your Photos & Videos to Mac/PC then they are safely residing there
4. You have uploaded some of it Cloud Drives using Files app (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud etc)
5. You have not imported nor you have synced nor uploaded your data to cloud drives prior to forgetting the Screen Time passcode then I am sorry, all your data is lost.