EV9 Outlook Client Required

This means that your Outlook Addin has detected that Outlook has, in it's profile, the tick box enabled saying that Outlook can connect to Exchange via HTTP.  Unfortunately the Addin can't tell whether you ARE actually connected via HTTP or not.. so we read some registry keys to check for the existing of this tick box :

If it's ticked, then we assume you might be.  This impacts the behaviour of the Addin.  One of the main things is that there is then a policy setting which can limit the functionality which is available when RPC/HTTP (aka Outlook Anywhere) is detected :

The message you're getting means your policy is probably set per the above screenshot (RPC over HTTP restrictions set to 'Disable User Extensions').  You'll need to change that, or untick RPC/HTTP in your Outlook profile.  If you change it in the policy, you'l need to synchronise the mailbox, and, restart Outlook.