Hi
There is no model of Apple Watch that has no metal or nickel touching the skin; the magnets in all models of Apple Watch contain some nickel.
If you are highly allergic to nickel, then is certainly possible that Apple Watch and/or a band may cause a reaction.
More information, including tips from Apple:
- Cleaning your Apple Watch - Apple Support
Includes "Tips to avoid skin irritation, or if you have allergies or skin sensitivities".
- Wearing your Apple Watch - Apple Support
- See under "Skin Sensitivities":
If you do experience redness, swelling, itchiness or any other irritation or discomfort on your skin around or beneath your watch or band, then Apple recommends that you remove your Apple Watch and consult a doctor / physician before wearing it again.