Your sysroot seems to be missing a header file:
/home/abhi/window/mainwindow.h:12: error: opencv2/opencv.hpp: No such file or directory
and your sysroot seems to be missing libraries:
/usr/local/oecore-x86_64/sysroots/x86_64-angstromsdk-linux/usr/libexec/arm-angstrom-linux-gnueabi/gcc/arm-angstrom-linux-gnueabi/7.3.0/real-ld: error: cannot find -lopencv_dnn
Your SDK does not contain the files needed to develop against opencv.
So did you:
Add what ever package you are going to use to the image recipe, E.g. added them in local.conf with something like
IMAGE_INSTALL_append = " opencv <whatever other package(s) you use>"
bitbake the image
bitbake the SDK with
bitbake <your image> -c populate_sdk
Install the SDK and configure qtcreator to use it.
Once the above is done and you can successfully compile, in order to run the binary on the target you also need to install the image on your module.