I can't buy Cubase?

Yes, the service Steinberg had been using is in trouble.

This affects Updates and Upgrades, which are mainly available only through Steinberg, new licenses can be bought from other retailers if you have access to those who carry it, like guitar center.

Steinberg’s licensing servers are working normally though,.

So you have a couple options.

What makes the most sense is to download the trial version from steinberg.net, which gives you a couple months of use.

Please see this topic and in regard to your trial period, this post in that same topic:

Please continue there, I’ll close this soon to avoid having multiple topics on the subject.