If, like me and countless others, you're coming to Jira Cloud after years of working on Jira Server or Data Center. One of the first things you might run into is the fact that your favorite old plug-ins (fine, apps, add-ons, whatever Atlassian is calling them today) you used to use to create workflow validators or conditions don't work quite the way they used to.
I'm talking about stalwart standbys like Jira Workflow Toolbox, Jira Miscellaneous Workflow Extensions, Power Scripts and ScriptRunner.
So here's the deal: Atlassian changed things up. Now the only way that plug-ins can provide validators and conditions is via a tricky language called Jira Expressions.
Luckily some add-ons provide handy templates (Cloud Workflows, and Power Scripts) that you can then tweak, or include a helpful editors and testers (Cloud Workflows, JMWE and JWT). But inevitably, you're going to have to get in there and figure it out yourself.
If you want to use custom workflow validators or conditions on Jira Cloud, you're going to have to learn Jira Expressions.
At its simplest, you might need to do something like "Only the Reporter should be able to close a ticket". So that expression would be:
user == issue.reporter
(That's from JMWE's Use Cases.)
Or maybe you need the old "Check for unresolved sub-tasks" condition before you let a user resolve a parent:
issue.subtasks.filter(s => !(s.resolution != null)).length == 0
(That's from JWT's Use Case Library.)
You can even check links:
.filter(link => link.type.inward == "is blocked by" ||
link.type.inward == "is mitigated by")
.length > 0
There's a lot you can do with Jira Expressions, and you're going to "get" to learn Boolean logic which is... some kind of fun. But for sure, the best way to learn is by doing. Check those links above, as it's very likely there's an existing "recipe" that can be adapted to your use.
But if you're really stuck, post here in the forums (tags: cloud, jira-cloud, jira-expressions), where some of us apparently have nothing better to do but than to debug others' Jira Expression issue. (Or in some cases like @David Fischer _Appfire_ maybe they get paid for it. :-)