chih what's your Composer version? You can find out with composer --version If it's too old (particularly below 2.0) you might need to update.
I've never seen that particular error, but it seems to reference the name commonly used for local extension development. Did you modify your forum's composer.json to add a development folder? If so it means you modified your composer.json but didn't create the folder you referenced in the custom repository definition.
