Block an assignee from having multiple issues in progress.

[alkber]

Well, i know , for some of you it doesn't make sense to block. However, our workflow has a constrain that an assignee can only work on single issue at a time. If he / she attempts to change state of another issue  assigned to him / her to IN PROGRESS . He should be blocked from doing so. 

There is a similar feature, for subtask, where parent task can be blocked from issue transition if subtasks are in some other state. We are using JIRA on cloud. Though we have found some plugins to do so, they are meant for JIRA server instances. We are not using JIRA Agile.

[Nic Brough -Adaptavist-]

I'm afraid there's no way to do this on Cloud - because most people can, and do, work on more than one issue at a time, there's nothing to support it in the core of JIRA, so you need an addon, and the only addons that do it, are, as you say, not available on Cloud.