I just started a cloud trial and when clicking on the install button for the AWS add on it gives this error message:
Error downloading update from https://splunkbase.splunk.com/app/1876/release/4.4.0/download/: NOT FOUND
No, the ones that must be installed by support have the install button greyed out. This let me click install, then gave the error. The issue is the version is incorrect. The version should be 4.5, but it is looking for version 4.4, which doesn't exist.
I would check with Splunk support on this. Do you also have the AWS App installed? The last time I was working with teh Splunk Cloud, I had to have Splunk Support do the installation of the Splunk App and it's Add-on.