If your puppy moves freely indoors without the lead but stops as soon as it is attached, the gear or its pressure may be the hard part. Do not drag them forward or let the lead trail unsupervised where it can snag. Check that the collar or harness fits and that the clip is not pinching fur or skin.
If your puppy has a necessary toilet trip, use a secure, comfortable way to get to the safe spot while you make a separate training plan. Ask your veterinarian about suitable outdoor places while vaccination guidance is still relevant. A sudden refusal, limping or sensitivity to touch merits a veterinary check rather than more coaxing.
At home, when nobody needs to leave, show the lead and reward a calm look. Practise the sound of the clip away from your puppy first. Then, if they are comfortable, attach it briefly while you keep hold of it and let them choose to move. Reward an easy step and unclip before they worry. If they freeze, make the next attempt simpler. The loose-leash walking guide covers skills after the lead feels comfortable, not a reason to pull a frightened puppy outside.
Today, inspect the fit and practise one easy glance at the lead away from the pressure of a walk. Moving one step by choice counts.
Sources: Dogs Trust on preparing a puppy's first walk. Dogs Trust on gentle harness introduction.