If You using forms (Start Custom form from the SharePoint List), prepare the validation:

1. On screen Visible
   set the default variable to show error

   UpdateContext({
      
       showValTask: false,
       showValSomeOther: false,
       ....
   });

2. Set the Text property for the error message
   
3. set error label
   Visible = showValTask
   

4. Hide the validation on the InputBox when do any changes
   OnChange UpdateContext({ showValTask: false})

    

5. Finally do the validation OnSave

   If(showValTask || showValDescription || ...... ,
    UpdateContext({locIsAllValid: false}),
    UpdateContext({locIsAllValid: true})
   );
   
   If(
       !locIsAllValid,
       Notify(
           "An Form entry is required or has an invalid value. Please correct and try again.",
           NotificationType.Error,
           4000
       )
   );
   
   If(
       locIsAllValid,
       SubmitForm(frmNew);
   	UpdateContext({locRecord: frmNew.LastSubmit});
   	Refresh(Submissions)  
   );
   If(
       locRecord.ID > 0,
       Notify(
           "Deliverable has been Updated!",
           Success,
           5000
       );
       Navigate(scrGalleries,  ScreenTransition.Cover);
       
   )