میخوام به انگلیسی و محترمانه بگم" نمیتونم از عهده ی فلان کار بر بیام "

به نظر من بهترینش  اینه .

Im not able to manage it  as proper as i should

Or

I cant manage it  

سلام

It is beyond my strength

اگر هم از نظر مالی نتونید از عهده چیزی بربیاید میشه گفت: 

I cannot afford it

Excuse me, I am unable to handle that task.

"I'm afraid I'm unable to take on this task at the moment.""Unfortunately, I don't have the capacity to handle this task right now.""Regrettably, I'm not able to fulfill this request at this time.""I must respectfully decline this task due to other commitments."

**"I'm currently unable to accommodate this task."