Why can't I input data in Excel on my iPad?

Microsoft Productivity Apps for iPad (such as Word, Excel, PowerPoint) offer limited functionality without an Office 365 Subscription - but only for devices with a screen size of 10.1” or less.

Devices with larger screens only permit you to view (but not create or edit) an existing document.  Full App functionality requires an active Office 365 Subscription.

And what iPad model do you have?

Settings > General > About > Model Name