An easy way to build AppSheet apps from Excel is to use the AppSheet add-on found in the Office add-in store.
While this process is mostly seamless, there are some specific behaviors of which you should be aware.
With Office 365, if a user has the spreadsheet open for editing, other users and third-party apps like AppSheet can't make updates to the spreadsheet. Consequently, if you intend to build an app that updates the data (as most apps do!), please make sure to close the spreadsheet first. Otherwise, you'll see conflict ("409") error messages and the app won't be able to save changes to the spreadsheet.
If using SharePoint, please note that AppSheet can only access files, folders, and subsites that are stored within the default root SharePoint site of a user's Microsoft account. All other custom sites and their respective files, folders, and subsites will not be accessible. Additionally, if the root site has been migrated to an on-premises server, it will not be accessible to AppSheet.
You can learn more about other data sources here.