Dear Loris.G.
Good day! Thank you for posting to Microsoft Community. We are happy to help you
I would like to check whether is your form shared externally? If so, the file upload option won't be available as files can only be uploaded if the form is restricted to only people in your organisation. This has nothing to do with the license.
If you want to share with a specific user, you can contact the admin user to try adding him to the guest user of your organization:
Guest users in the Microsoft 365 admin center - Microsoft 365 admin | Microsoft Learn
Thanks for your cooperation.
Sincerely,
Tin | Microsoft Community Moderator