The message can happen as in Joomla tables a field "metakey" is used, but in JEM we're using a field "meta_keywords". If you did install JEM2.3.0 it might be due to some other plugin/component/etc..
One thing that can be done is to add the field "metakey" to resolve it, but that can lead to other problems when you're going to update to newer versions of JEM later on. Besides of that it's good to troubleshoot to know what's causing the issue.
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