From what you've posted.. are you actually including a valid signature? If you just have 'postingkey' in that place it won't be a valid transaction.

Most people use a library to do the ECDSA. I suggest dSteem. Or soon to be dHive? :)

If you need more help, I suggest joining one of the many community discords and jumping in a dev channel. I'm sure it will be easier to get live support there.