I first tried to generate GNAT Studio Ada compiler 26.0w completely clean (empty directory. It seemed not to have gprbuild inside it. Then I first toke 20.0w (also complete) which seemed to have gprbuild inside. The I generated 26.0w on top of 20.0w. It seemed to work and I could of a rather complicated program see that all ads and adb files were translated but then the message “cannot generate code for file gtkada-intl.ads (package spec)” appeared. What does it mean and how to solve the problem.
It means what it says no code can be be generated for a package specification. It is not a problem it is a statement of fact.
As for the actual problem, you have either GtkAda not installed or broke it. GtkAda sources are never [re]compiled.
Assuming that you installed the GNAT FSF build for GPSStudio form the Github Alire-dev site, you need to install (unzip in this case) also the GPRBUILD and GNAT releases. These last two contain Gprbuild and the various GNAT tools (Gcc!) needed to create Ada programs. As for Gtkada, you must either fallback on the latest self-installing exe (2021 I think) or build Gtkada from source. In the latter case you need the MSYS2 environment.
See also this post Building Gtkada. Good luck!