Can you please check the logs under <confluence-home-directory>/logs? The file is called atlassian-confluence.log
In case you are not sure where the <confluence-home-directory> is located, please refer to this documentation: https://confluence.atlassian.com/display/DOC/Confluence+Home+and+other+important+directories
This should tell us more about the error so we can try to help you.
Can you also check your base URL?
If the Base URL you have is different from the one you are using to access your Confluence then the add on will indeed fail to be installed.
If the base URL is also correct then let us know your JAVA and Confluence version.
Don't forget about the atlassian-confluence.log file .
Thanks and Regards,
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