I have spun up a vRA and vRO Lab quickly to perform some testing recently and came across the above error when trying to at my vCenter endpoints into vRealize Automation.

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You need to deploy the agents for each of your vCenters,  To do this, connect to your VRA Appliance to pull down the Installer

https:// <VRA Device name or IP>:5480/installer

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Download the IaaS Installer (I did this from my Iaas Server)

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Run this as Administrator, Next through the .NET warning if you get one and Accept the EULA.

Connect to your vRA Appliance with the root account and password and select Next

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Select Custom Install and Proxy Agents > Next

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Enter the credentials, in my case they were the local admin credentials as I was using my Lab

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In the Agent Type Drop down – select vSphere
Enter and Agent Name
Enter the Manager Service Host and Model Manager Web Service Host – For me, these were my IaaS Server
Add your vCenter Server and click Add

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This will drop the Endpoint name down

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Select Next and on the following screen Install

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The Installation will run through and complete

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I did this for both my vCenter Servers.  I was then able to add both of these as Endpoints in vRA

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