Upwork approves profiles that have a strong portfolio. The best way to do this is by creating a website and putting your portfolio projects there. The portfolio is important because it lets them verify whether you are a real person and whether your skills are adequate. You can show off your portfolio by creating specialized profiles.
You have the option to Add specialized profile in the profile editor (Find Work › Profile). To see a preview of how your specialized profile will look to clients, select View. - Upwork Support
For example, my profile was rejected the first time I applied on Upwork. Then I researched the subject and then brought the domain sakibarifin.com and posted some articles there. I then set up my Upwork profile and created a content writer specialized profile. I applied again and this time my profile was approved.
The website doesn’t necessarily have to be a paid one. You can create static websites on Github pages using Jekyll and get free domains like .tk and .ga. Alternatively, you can set up your portfolio in websites such as Github (Coding), Stack Overflow (Programming, Development), Medium (Writing), Behence (Design) etc.
Please remember that you need to be at least 18 years old in your KYC documents (that is your National ID or Passport) to earn on Upwork.