What kernel snap and revision are you using?
If the resulting image boots and sets up itself correctly you are fine. Assertions are signed documents carrying policy and metadata. We need to pull ones with formats compatible with what is supported by the snapd version that is put in the image. The kernel initramfs has also some code that needs to check some of those assertions, recent UC20+ kernels ship some information to know what they support so that ubuntu-image can pick supported formats combining that information with the snapd one. But if the kernel does not ship this information, ubuntu-image can only use the snapd version/provided information, and the resulting image might not boot.