As you mentioned, you are running 1.18.4 version of kubernetes. This version is not fully supported version in AKS (it is available in preview mode and not in GA mode) and hence you are seeing incompatible version in Advisor. This version would be available in Aug 20 as GA. Please check the release schedule for AKS information here.
Since this is preview mode, you will not be able to downgrade to 1.17.x (supported AKS GA versions). If you are planning to use AKS in production environment, I would recommend you to create another cluster with 1.17.x and move your workload to that cluster as that is fully supported version in AKS for now.
Hope this helps.
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