As outlined in Use the JIRA DVCS Connector Add-on, You can connect Bitbucket Cloud or GitHub accounts with JIRA applications using the free JIRA DVCS Connector add-on:
This add-on comes bundled with JIRA Software. For JIRA Server versions earlier than 7.0, this is a system add-on that you can install. For more details, see Linking Bitbucket Cloud and GitHub accounts to JIRA Software. [emphasis mine]
So you need to make sure that your Atlassian Cloud subscription is using a JIRA Software license (rather than just JIRA Core). Within JIRA Software, you can access the DVCS Connector configuration via the following menu paths:
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