I deleted a message on my Apple Watch and it deleted it all n my iPhone how can I get the message back

Since they deleted on both your Apple Watch and iPhone, this indicates they are iMessage messages (meaning with other iOS based devices). Assuming you have a current model iPhone, with the most up to date software (current iOS version is 16.4.1), open the Messages App on your iPhone and tap Edit in the upper left hand corner of your iPhone screen and tap show Recently deleted if it is there (which would only be there is there are any recently deleted messages you can see). If there are none, the messages are gone and can't be recovered.

Not iMessage messages. Once deleted on one device, they delete on all devices.