Project Online and Project Server are Enterprise project management tools that are a central repository for schedules, resources, and timesheet management.  Project Server is the on-prem version and Project Online is part of the Microsoft Office 365 arena.

Consider the following scenario in Project Online or Project Server:

When you click on Timesheets to enter your time, you receive the following message:

We can't display the select time period.  It might be closed or your administrator hasn't create it yet.  Please try a different date.

Excel is a tool that can be used for reporting on data in Project Online via OData and Project Online's API.  If you want to read the data from Project Online, it must be set to allow you to access and read the datasets.  If permission has already been enabled, you'll need to enter your credentials. 

If you use Project Professional Desktop Client and your organization has Project Online or Project Server, you'll need to do this one-time configuration to set up a profile/account.  Setting up a profile tells the client how to reach Project Online to publish your schedule.

Project management apps and software offer various features that historically deliver extraordinary results to a business. Namely, project managers benefit significantly from garnering a firm handle on their people, ongoing work, and overall objectives.