Sometimes you might want to compose a module from multiple smaller modules using either iguazu-holocron or RenderModule. However, if you have not added the module you are trying to load to your module-map, it will fail to load.
Furthermore, if you are calling RenderModule but not calling composeModules in loadModuleData, this will result in a warning stating that the module is not in the module map.
If you are getting this locally while using one-app-runner, you need to make sure that the module that is missing is served in the one-amex section of the module you are running npm start in.
If you are getting this in an environment that you are deploying modules/One App to, you need to make sure that you have added the required module to your module map that you are supplying to One App.