To my knowledge, Excel iPhone does not have a built-in data validation feature. But it's been a while since I've dealt with it, something might have changed...or not. The Excel app for iPhone is primarily designed for on-the-go viewing and editing of spreadsheets and may not offer all the features available in the desktop version of Excel or the iPad version.
However, Microsoft frequently updates its apps, so it is possible that data validation or similar features may have been added to the Excel app for iPhone since my knowledge cutoff. I recommend checking the App Store for any updates or reviewing the app's documentation for the most up-to-date information on data validation capabilities.
Alternatively, you may consider using Excel on a desktop computer or an iPad, where data validation features are more robust and readily available.
To get the most accurate and up-to-date information about Excel for iPhone and its features, I recommend visiting the official Microsoft Excel for iPhone help