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.
For more information about how to Upgrade to Business trial, please follow the steps in the Start 30-days Business Trial online help article.
Then, save your projects to your Business Drive 365 folder or to your local device. After all your projects were saved on Drive 365 or local device, you can create a master plan by adding the subprojects from menu Project – Subproject – Drive 365 or Computer.