Nope. Sorry.
A tenet is one of the principles on which a hypothesis or a theory is based. There are no hypotheses and/or theories supporting the validity of astrology. Ergo no tenets.
And why am I not the least bit surprised to see that a flat earther believes in the notion of astrology LOL.
You are an idiot. This is what is called the definitist fallacy. The thing that is in dispute is not whether there are astrological tenets (in the way that you define them), which you tacitly assumed existed when you said:
"The tenets of astrology can only exist in the minds of the believers. "
other wise what does "only exist" mean here?...erm...not exist?
So you are denying that astrological tenets can exist at all, and not that they just exist (or "only exist") in people's minds (which is the thing in dispute), but rather that astrology does not have any tenets, because tenets are "hypotheses and/or theories supporting the validity" of some theory, now to go along with your new claim (by redifining the word tenet", which is wrong anyway because a tenet is a "principle or belief, especially one of the main principles of a religion or philosophy" it does not necessarily have to make the theory valid.), you have just defined astrological tenets and lots of other tenets out of existence, which is not what you were trying to argue, and so you have moved the posts of your argument, which you keep doing, in order not be refuted - they are called conventionalist ad hoc stratagems.
The conclusion here is that
astrological tenets cannot exist, but they only exist in people's minds
they do exist and they don't exist [p ^ ¬p]