String does not match expected pattern

Hello generous community.
I m running into an error when i create an extension.
It gives this error:
POST http://localhost:9486/1.1/extensions: [“exten: [‘String does not match expected pattern.’]”]

These are the snapshots:

Hello, an extension is only a number, why do you want to add an extension from this form, it’s for very advanced part.

create it from the user directly !
easy !