I use Mac and QGIS 3.4 and got the same error.
After checking the PyQt5 folder in QGIS, I found there is no QtChart file in the folder, which is the reason why we cannot import it.
And the version of PyQt5 in QGIS is 5.11.3
The folder for PyQt5 in QGIS is
I have an anaconda version of PyQt5, which can import QtChart, the version is 5.13.1.
I tried to copy the QtChart.so file from the anaconda PyQt5 folder to QGIS PyQT5 folder, but it doesn't work.
Check @Cody-chen method from the link below to see if it can solve your problem.
I use pip to install PyQt5 for the system version of Python, which turns out it also doesn't include QtChart.
python3 -m pip install PyQt5==5.13.1
After that, I use
python3 -m pip install PyQtChart, then system Python can import QtChart, so I think pip install default method won't install QtChart.
Though QGIS on Mac uses the system version of Python, and it can import QtChart. But I still cannot import it in QGIS, because QGIS has its own PyQt5 folder. I haven't find a way to solve that. Sorry.