Be out of your depth
Definition 1 of 2
Pro Tip 1/3Skill Level
Use when admitting a task is beyond your current skill set. 
I'd love to help with the budget report, but I'm out of my depth with numbers.
Pro Tip 2/3Emotional State
Reflects feeling uneasy or insecure about being unable to manage a task. 
In the debating competition, I felt out of my depth and anxious.
Pro Tip 3/3Humble Admission
Shows humility by acknowledging a lack of expertise. 
I'm out of my depth when it comes to French cuisine.