The 2 reasons that the Share button won't be available are:
As the Share Page add-on is enabled, the problem lies in your outgoing mail server. Go to Confluence Administration > Mail Server and follow the instructions to check your mail server is setup and working correctly. Is everything there working as expected?
If the Share Page add-on is enabled and the mail server is setup correctly and sending mails then the Share button should be visible.
As it's not, your next step will have to be to log a support ticket with Atlassian and ask them to investigate. I'm sorry I couldn't help you fix it. When Atlassian resolve this for you, can you post the resolution here so we can help other people with the same problem in the future?
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