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.

According to an Apple forum answer, it may be possible to edit files on iPad or iPhone just by signing in to a Microsoft account, even without a Microsoft 365 subscription, with some exceptions. Without signing in, the files are treated as read-only.