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! My name is Eoin and I'm a product manager working on Team-managed projects in Jira Software Cloud. We're continuously improving and updating Jira Software and feedback from our customers is ...
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