The Game Boy Advance version of The Sims 2: Pets does not contain a Pet Training career. That career track exists only in the console (PS2, GC, Xbox) and PC versions.
The GBA cartridge (32MB) lacked space for the full career system, 3D assets, and AI of the console/PC builds. It's a distinct, simplified life-sim tailored to the hardware.
Enjoy the GBA version for what it is: a portable pet-care sim with charming sprite work.