When a form is not able to create an issue, it usually due to one of two problems:
The user does not have create/edit issue permissions in the project;
Or there are required fields on the issue create screen that do not have a corresponding linked field on the ProForma form. In order to create an issue, ProForma needs to provide values for all mandatory fields. If the Create Issue screen requires a field and there is no corresponding linked field on the ProForma form, then you will receive a 400 error. Alternatively, you may receive an error telling them at a specific Jira field (for example the Description field) is required.
To resolve this issue, ensure that there is a linked ProForma field for each required field on the Create screen. It is also recommended that any mandatory Jira field be made mandatory on the ProForma form.