Very late response, but I was having the same issue with cubemx v6.14.0 working with stm32h755zit3u on a nucleo-h755zi dev-board and using stm32cubeIDE v1.8.0. After some investigation, cubemx is not updating the links .project file in cubeIDE. The .project file is just xml, so manually edit it and add links to stm32h7xx_hal_adc.c and stm32h7xx_hal_adc_ex.c (substitute h7xx for correct uC version for your project).
Possibly, I have set up something incorrectly in cubeMX - I'm just learning ADCs. But, re-hitting the "generating code" button in cubeMX did not wipe out the my .project edits. So, at least a temporary fix.
