It’s important to provide your NI number whenever possible. Your NI number is used to make sure you are taxed correctly on any earnings, and it means you will be making the correct NI and pension contributions. Not adding your NI can cause issues when it comes to things like tax rebates.
You can apply for your NI here:
https://www.gov.uk/apply-national-insurance-number
You can sign up without providing your NI number - but try to add it when convenient (you can do this in the “Payroll details” section of your app (Account → Payment details → payroll details).