Qt Creator's qmlls integration currently does not support multi-module projects. It says:

qmlls could not find a build directory, without a build directory containing a current build there could be spurious warnings, you might want to pass the --build-dir <buildDir> option to qmlls, or set the environment variable QMLLS_BUILD_DIRS.

Consequently, qmlls cannot find and import the project modules.

Workaround

• Disable qmlls, OR
• Set the environment variable QMLLS_BUILD_DIRS before launching Qt Creator (only feasible if the user is not working on multiple projects simultaneously)