How can I use Master project and subprojects feature on MPP files?
In order to use the Master project and subprojects feature, you need to upgrade to business trial from Online Portal if you do not have a Business Subscription.
Just Sign in with your email and password in Online Portal page then click on Upgrade to Business button: https://www.projectplan365.com/ups/Account/SignIn.
Then, open Project Plan 365 application and Sign In with your email and password (look upper right corner for Sign In button). Save your projects to your Business Drive 365 folder.
After all your projects were saved on Drive 365, you can create a master plan by adding the subprojects from menu Project – Subproject – Drive 365.