In a project, I've got several issue types, but I need to only have one issue type in creation.

[Nic Brough -Adaptavist-]

You can't do it natively - the whole point of the issue type is to allow people to choose a type.

There's a set of dodgy hacks at https://answers.atlassian.com/questions/179076 (I can never recommend the use of javascript on a JIRA front end, it's almost always a disaster)

[Taha Khanzada]

Hi Jen,

You can Create custom Issue Type Scheme for your Project and add only one issue type to this scheme. Probably you will need to add one sub-task as well.

Taha