Hi @Sabrin,
Thanks for reaching out!
Assuming you are referring to this tutorial, the differentials.qza file mentioned in there is produced from the community plugin q2-aldex2, which can be found on the QIIME 2 Library, also linked below:
https://library.qiime2.org/plugins/q2-aldex2/24/
I'd recommend reading over the above tutorial on q2-aldex2, which will help you to better understand how to install that plugin, and generate the differentials.qza artifact on your end.
Hope this helps!
Cheers 