@viko Happy New Year!
The dtbo.img contains part of the device tree, and as such it is important. However, during porting it is often not necessary to change that information, and in those cases the file is not provided. If the file is not in the artifacts downloaded from GitLab, then most likely it is not needed.
(Disclaimer: that is my experience from my own port.)
Do you look at generic install instructions, or instructions specific to the port?