Hi Jonata - Welcome to the Atlassian Community!
You can create an Automation For Jira rule that uses a Schedule component to run at a certain time.
Then add the JQL you want to run it against (basically a filter).
Then add a component to the rule for Edit Issue.
Select the Labels field and just leave the This field will be cleared option in place.
What were you doing when you got the error? Here is a better guide to get to Automation For Jira
Now when each issue is created, it will add the desired default label as long as the issue is created in that project. You might need to refresh the page after each issue creation to see it.
You can do this via global rules to apply to all projects also. You'll need to be a Jira Admin/Site Admin, then instead of going to Project Settings, go to Jira Settings (cog icon on left-hand navigation bar) > System > Automation Rules (left-hand menu).
For more on using Automation For Jira see these help pages.
For JSM June Challenge #2, share how your non-technical teams like HR, legal, marketing, finance, and beyond started using Jira Service Management! Tell us: Did they ask to start using it or...
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