Hey boobalan24,
We understand you're having an issue using Siri to send a WhatsApp message, however it's unclear if this is specific to WhatsApp or if this is occurring with Messages as well. We ask as it'll be important to isolate this issue further and determine if this is only happening with a 3rd party app or not.
If this is specific to WhatsApp, make sure the app is fully updated. If it's not specific to WhatsApp, make sure you've restarted your iPhone and gotten it fully updated to iOS 16.1 and tested further. If you're connected to Wi-Fi, test over cellular and vice versa.
If this behavior persists afterward, we'd suggest getting in contact with Apple Support so they can look into this matter with you further: Get Support
We hope this helps out.
Take care!