Hello 天宇 李,
Even though you are using Ubuntu 16.04, please install all the packages listed on the documentation for Ubuntu 14.04.
$ sudo apt-get install libsdl1.2-dev xterm sed cvs subversion coreutils texi2html \
docbook-utils python-pysqlite2 help2man make gcc g++ desktop-file-utils \
libgl1-mesa-dev libglu1-mesa-dev mercurial autoconf automake groff curl lzop asciidoc
$ sudo apt-get install u-boot-tools
And then try baking again.
As for performing a clean, which sometimes helps, you can do this by running:
$ bitbake -c cleanall fsl-image-qt5-validation-imx
And then you can bake again. Please keep in mind all code would be downloaded and compiled again, which will take time.
I hope this helps!
Regards,