I know this is late, but hopefully it can still help others as I ran into it today and figured out why it happens!
TL;DR - The Reporter field in your project configuration is Required. Update your project configuration(s) to have the reporter field be NOT required.
This is what Power Automate has in the documentation:
The Create a new issue (V2) action supports only simple field types such as 'string', 'number', 'date', and 'datetime' in the dynamic schema. If project configuration has a required fields of complex data types, the operation will fail with the error: "Field '{key}' of type '{type}' is not supported". To workaround this, please change project fields configuration and make these fields not required.
When adding the step in the flow, Power Automate hits up Jira to pull in some of the field information. It immediately finds that the Reporter field is required. PA would want to display that field and accept a Reporter entry, otherwise Jira would reject the new issue.
HOWEVER, the Reporter field is a type called 'user', which is not a string, number, date or datetime, which are the only ones supported by the action. Then, Power Automate doesn't know what to do with the field because it can't handle it, and so results the error.
To bypass this, make the Reporter field in Jira not required for the project(s) you want Power Automate to create tasks in. Do it for Epics, too, if you want to be able to create Epics.
NOTE: This also happens if you try and choose Sub-task for the issue type. Sub-tasks require a Parent Issue, and this field type is called 'issuelink'. Sure enough, 'issuelink' is also not a string, number, date or datetime 🙄. Haven't figured out a workaround for this one out yet, unfortunately..