If you have been online and you have seen that Alhambra tickets are sold out, don't worry.

The best page about this subject is here

https://infoaboutspain.com/article/alhambra_tickets_sold_out

First of all - Try again now
All the ways of buying tickets in advance appear here on this often updated page.
https://granadainfo.com/alhambra/advance.htm

Why do tickets become available?
First of all most tickets are given to third party operators such as guided tours, hotels etc
These companies release unsold tickets back into general circulation.
Also there are cancellations.

So just go back every day and it is very likely that if you are persistent you will get a ticket.

If you are impatient or you want to buy a ticket now so you can make plans.

Go and buy the Granada card which includes the Alhambra tickets. It makes sense if you are in Granada for a few days and want to visit several monuments. Otherwise get a guided tour.
See full info here
https://granadainfo.com/alhambra/bono.htm

Get a guided tour
If you don't mind paying a little extra just get a guided tour which includes tickets. Just turn up at the meeting point and let other people decide everything. There is an excellent multi provider page here which all the guided tour companies use.
You could have everything booked and sorted within 10 minutes.
https://granadainfo.com/alhambra_ticket_search.htm

Alhambra FAQ
For almost all questions about the Alhambra and getting tickets look here.
https://granadainfo.com/alhambra/faq.htm