Besides S4 extension tool in Fiori UI, you can also create extension field in backend using report CFD_TEST.

Caution

(1) NEVER run it in development system like ER9 and AG9 currently! If you want to test your extension CDS view for POC, contact me first.

(2) Avoid to create too many extension fields which might have negative impact on system performance.
Run report, specify mandatory field as example below.

As I mentioned in this link, the extension field generation is done in ASYNCHRONOUS way - technically speaking, it is finished in a background job automatically scheduled and released via extension field framework.
You can debug to get the exact job id:

Sometimes it takes long time to get your extension field generated and published, it is simply because there is no more background job processor with status "Free". In this case you can do nothing but wait until other job is finished.

How to check your field generation job status

select the job and click button "Job log":

Here you can monitor job execution detail:

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