I recently worked on updating automation in Jira and encountered this issue. I followed this knowledge base article: https://confluence.atlassian.com/jirakb/unable-to-save-the-automation-rule-1332805879.html.

Steps I did for context
I removed the extra projects in the project scope in a separate Jira Chrome window. I ended up having to refresh my draft automation Chrome window, losing my work in the process window, to be able to hit the Update button and save.

To resolve this issue, I ran into the following problems with the application:
1) There is no warning or error message describing why the Update button wasn't working.
2) I had to refresh my draft automation Jira Chrome page to be able to click on update and save my work. Thereby losing my work and forcing me to rebuild it.

I have the following suggestions for feature/usability improvements.
1) When a draft automation can't be saved after clicking the Update button, a warning or error message should be displayed.
2) A wizard or dialog box should allow the admin to change the automation project scope or help resolve other errors on the same draft automation page. Thus, work progress should still be present.
3) Make it so that updating and saving the draft automation page does not require a page refresh.
4) Drafts of the automation should be saved automatically, similar to how they work in the Workflow Schemes section of Project Settings.
5) If there is an error, the automation page should gracefully fail and tell the admin the problem and ways to resolve it on the same page. I shouldn't have to find the knowledge article in the first place.