At the beginning of your deletion script, add the step "Set Error Capture [On]"
Perhaps this will help?
Defining field validation
see especially, #8
To display a custom message if validation fails, select Display custom message if validation fails. Type a message (up to 255 characters).
We developers have been abjectly groveling at the feet of FileMaker Inc. for the last couple of decades, begging them to give us more control over the Custom Dialog script step. All I can suggest is that you go to the "Product Ideas" forum, weep tears of sadness, and throw yourself on their mercy. Granted, this has never worked before, but maybe you'll be the one who finally tugs on their heartstrings enuf to finally make it happen.
Hi , Thanks But I Want To Know That How Can I Change That Dialog ?
There are standard dialogs that you can suppress. You can change the deletion script to both hide the regular dialog and to pop up a custom dialog. For field validation, you can provide an alternate message, as Beverly as pointed out. Or using OnValidation field script triggers, you can add various error checks and corresponding messages to your heart's desire. That is three different methods for which you'll find more instructions in FMP's help files and elsewhere online. I agree with Richard that custom validation messages right in the field definition would have been preferable and are long-desired, but we have other methods.
If you have additional questions about FileMaker, this particular forum is not where they belong. The FileMaker discussion forum is at Discussions
There are some great suggestions above. I am going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive even more views and potentially more advice!