Hi @Ivan Hohlov ,
Just like Nick pointed out, you may be restricted because of a recent backup, so you'll have to wait the cooldown. More info here: https://confluence.atlassian.com/jirakb/jira-backup-frequency-is-limited-you-can-not-make-another-backup-right-now-779160825.html
Another possibility is that you have Next Gen projects in your cloud instance. These can't be included in the backup for server as server doesn't have Next Gen, so you'll have to move the issues to standard projects first and create an export for server after.
In both cases, you should see some kind of message indicating as of why you aren't able to create the backup.
This happens if you have recently taken a backup. The process is quite intensive, so Atlassian only allow it to run once every two days.
Someone mentioned 47.5 hours to me a while ago, but that is third-hand, so assume the official line at https://confluence.atlassian.com/adminjiracloud/exporting-issues-776636787.html
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