Some things to bear in mind with regard to your question and RIoTboard:
1. You didn't mention exactly which Yocto version you are attempting to use or what procedure you are using when trying to build. Like any piece of software, Yocto changes over time, so what works for one release version may not work for another, and it is important to give some details on exactly what you are trying to do. Here is one example of a change that may make older build procedures invalid (or at least require systematic changes in order to work properly), if you are trying to use a more recent version of Yocto:
2. Even if you give additional details, there is no guarantee that someone can or will answer your question. If, for example, you are trying to use the very latest version of Yocto, you may quite literally be the first person to attempt to build using that version, not just on this board, but anywhere. RIoTboard is past its heyday, and there are not as many people available to answer questions about it as there were a few years ago.
Good luck with the build!