It seems you don't have create permission even after the registration)
Normally the link is https://avono-support.atlassian.net/secure/CreateIssue!default.jspa
I would recommend you to leave a comment here:
https://avono-support.atlassian.net/wiki/display/PUML/Confluence+PlantUML+Plugin with the info that you cannot create support ticket after registration.
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