"The latest versions of Tempo Folio for JIRA 6.4 and higher"
https://tempoplugin.jira.com/wiki/display/JTS/Latest+Releases
Maybe your JIRA version is not compatible?
If you are on Cloud, it should appear once you've subscribed, although you may find you need to give it a few minutes to become enabled.
If you are on server, you need to install it as well as get the trial licence.
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