I need to restore my iPhone 12 Pro as I lost my old ph but not able to restore

When setting-up your replacement phone, you’ll be prompted to restore from a backup (if available).

Select the appropriate “restore” option for your circumstances and follow the prompts.

See more detail here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

No harm done.

Simply reset the phone - skipping the “backup” step - and start over.

Recovery from a prior iPhone backup can ONLY be accomplished during “Setup.”

How to factory reset your iPhone, iPad, or iPod touch - Apple Support

The “completeness” of any data restoration will be as good as the “completeness” of your prior phone’s backup and/or iCloud sync.