To create a Ubisoft account, users need to be above the minimum age set in their respective countries (for example: 13 years old in the United States and Canada, 16 in most European countries). This means that you won't be able to create a Ubisoft Account if you use a Nintendo Child Account.
If you encountered the error message shown in the following picture or if nothing happens after pressing on Create a Ubisoft Account, it probably means that you are in this situation.
Please note that Ubisoft is committed to complying with the privacy laws and regulations of all the countries where it operates, notably the General Data Protection Regulation (GDPR) in Europe and the Children’s Online Privacy Protection Act (COPPA) & ESRB's Privacy Certified Program in the US. Moreover, the ESRB reviews our products and services to make sure they are compliant with US privacy laws.