Hi @hiti :
Thanks for @ChrisPiasecki 's suggestion.I want to use a few sentences to summarize what I want to add.
1\To achieve this function, two steps are required: verification and error reporting
2\To verify that the user's input is correct, you can set the OnChange property of this control. (When the user input is completed, the OnChange property will be triggered)
3\To pop up an error message, the easiest way is to use Notify function in Power Apps function.
I've made a test for your reference:
I assume that if the user enters characters other than numbers, the application will report an error:
Set the text input control's OnChang Property to:
If(!IsMatch(Self.Text,"\d*"),Notify("You can only enter numbers!"))

Best Regards,
Bof