If your JIRA is version 5.0.7 and below, you might encounter the known problem discussed at https://confluence.atlassian.com/display/JIRAKB/The+Atlassian+Marketplace+server+is+not+reachable.+PacException%3A+404
Otherwise, most likely you have connection issue between JIRA server and Market Place. For troubleshooting guide, you may refer to https://confluence.atlassian.com/display/UPM/Problems+Connecting+to+the+Atlassian+Marketplace
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