From your descriptions I understand that you are not using the latest meta-variscite.
Anyway, if the ubi image is not created add the following line to the conf/local.conf file under your build directory to attempt to create it:
IMAGE_FSTYPES_var-som-mx6 = "tar.bz2 ext4 sdcard ubi"
However, it will likely fail during build, because the fsl-image-qt5 image is too large to fit in the NAND flash which is configured by default to be 512MB.
If you have a 1GB NAND, you can change the UBIFS parameters in conf/machine/var-som-mx6.conf under the meta-variscite directory, by commenting out the 0.5GB parameters and uncommenting the 1GB parameters.
If you have 0.5GB NAND, you should either just use the eMMC for the filesystem (which is recommended anyway), or try to remove packages from the fsl-image-qt5 recipe for it to fit.