FIeld configuration schemes are not available if you are actually using a next gen project in Jira cloud. In case you are using a classic project you can assign the field configuration scheme to a project like it is outlined in the article which link Carlos posted above.
Probably you are looking for Screens (organised in Screen Schemes) instead?
The documentation is to be found here:
Hi All! We’re excited to share the launch of an announcement banner that lets Jira site administrators communicate directly to their users across their Jira Cloud instance. ...
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