I'm leaving end of this month.

[B45]

I'm leaving end of this month.

VS

I'm leaving end of the month.

Are both okay?

 

[Veloviv]

No. It is correctly written as I'm leaving at the end of this/the month.

'At' expresses arrival. In this case, it is arrival to the end of the month.

 

[Raymott]

"I'm leaving at the end of the/this month" are both correct. Neither of your sentences are correct in standard English.