Portfolio for Jira installed (on Jira Core) but not working
Portfolio for Jira could not find software necessary to run.Please install or update to Jira Software (7.0.5 or higher).If you need help installing software or think this message is wrong, please contact your Jira administrators.
IF Add-on not compatible with Jira Core why is it displayed when you search for addons.
Jira Portfolio will function in a limited capacity with just Jira Core installed, but access to the Live Plans feature is not available. Additionally, you can install JIRA Software without a license to resolve the error that you're seeing. Having said that, the bulk of features provided by Portfolio are geared towards the agile functionality that Jira Software provides. The features provided by Portfolio may not be useful in your environment if you're not Jira Software.
A suggestion for the Marketplace behavior you've encountered has been raised here https://jira.atlassian.com/browse/JPOSERVER-1606. If you've already purchased a license and would like us to issue a refund, you can make that request here https://www.atlassian.com/company/contact/purchasing-licensing . We certainly don't want you to pay for something that isn't compatible with your configuration!