When following https://developer.garmin.com/connect-iq/sdk/, I observe the following error during plugin installation:
An error occurred while collecting items to be installed
session context was:(profile=D__eclipse_cpp-2020-09_eclipse, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
No repository found containing: osgi.bundle,IQ_AppSettingsEditor,3.2.3
No repository found containing: osgi.bundle,IQ_IDE,3.2.3
No repository found containing: osgi.bundle,IQ_PackageWizard,3.2.3
No repository found containing: osgi.bundle,IQ_SdkManager,3.2.3
No repository found containing: org.eclipse.update.feature,connectiq.feature.ide,3.2.3
No repository found containing: org.eclipse.update.feature,connectiq.feature.sdk,3.2.3
According to the SDK manager, the 3.2.3 SDK is installed and selected.