Thank you for reaching out to us.
Does your account has required Sentinel RBAC permissions to assign incidents ?
Reference :
https://learn.microsoft.com/en-us/azure/sentinel/roles#:~:text=Microsoft%20Sentinel%2Dspecific%20roles
https://learn.microsoft.com/en-us/azure/sentinel/roles#:~:text=to%20another%20resource.-,Microsoft%20Sentinel%20roles%20and%20allowed%20actions,-The%20following%20table
Let me know if you have any questions.