Hi @andreisv,
If the links are correct, it seems you are generating the initial image that you flash into your raspberry using the Debian-based system RaspiOS (first link) and the mender-convert tool. For generating the artifact, you are using the tutorial created for Yocto-based images (second link). If that is the case, both workflows are not compatible between them.
If you use RaspiOS and mender-convert (first one), you have to follow this other tutorial: Raspberry Pi 3 Model B/B+ Raspbian, and it will generate a compatible artifact (with *.mender extension) that will work out of the box with your current system.
If you want to stick with the Yocto workflow (second link), you need to generate only the Yocto-based images/systems and you don’t need to use the RaspiOS system you downloaded initially.
Hope it helps to clarify. Have a great day! 
Luis