Description Resource Path Location Type Unknown referenced nature: fr.ac6.mcu.ide.core.MCUProjectNature. .project /bootloader Unknown Unknown nature Facing this error as attached.

That is a warning, not an error.

Does the IDE build the bootloader if you modify one of the source files? What if you do a "clean" and build? Or do you always get the "nothing to build" message. If the bootloader DOES build when it supposed to, then perhaps just ignore the warning (not an ideal solution).

Which IDE is this? System Workbench? Or CubeIDE on a project imported from System Workbench?

I will not be much help, but that may give people with more in-depth Eclipse knowledge the info they need to help you.

I cleaned the project. Still getting this warning.

This is on stmcubeIDE version 1.9.0.