I cannot edit Excel file in Excel for Ipad - Convert buttons does nothing
As noted in the comments, editing is only allowed if one has a subscription to Office 365.
As noted in this article Excel for iPad is free as a read-only client for Excel spreadsheets.

Alternatives to purchasing a subscription to Office 365 are available but each have their drawbacks.  (Formula editing, format retention, etc.)


Google Sheets [App Store link] offers free editing of Excel files.  See documentation for details.
Apple's own Numbers for iPad will open Excel files for editing but some of the functionality is limited.