Gerrit code review - What does "cannot merge" mean on the bottom right?

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To quote the Gerrit documentation:

Submitting a change may fail with conflicts. In this case you need to rebase the change locally, resolve the conflicts and upload the commit with the conflict resolution as new patch set.

If a change cannot be merged due to path conflicts this is highlighted on the change screen by a bold red Cannot Merge label.

There have probably been changes made to the Wireshark code since the last patch set was submitted; the last patch set for that change was submitted a year and a half ago. Mr. Jalali needs to rebase against the current Wireshark master branch, fix any conflicts (for example, if some of the code he changed was also changed in the Wireshark repository in ways that cause his change not to match), and re-submit the result.