Private Key in HD Account Bither Watch Only I have a watch only HD account in Bither and I've got a password of the wallet but I don't have a private Key. I need to send  BTC received in that account to another address in another account
If you don’t have the private key, you cannot spend the bitcoin in a watch-only wallet. There is no way to find, calculate, or recover a private key for a given address. 

After all, if you were able to do that, Bitcoin, and many other systems that use public key cryptography, would be broken.

Beware: some scams involve a scammer telling you to pay for access to a private key for an address that was set up as a watch-only address. If this sounds like the situation you are in, so not send the scammer any money, you will likely receive nothing in return.