How to flatten such part ? Can anyone make flat pattern of this?

It is usually not easy to create a flat pattern in Creo from swept blend type of a transition in sheetmetal...

Especially if the transition was modeled as a solid part and then sheetmetal conversion was performed.

Try starting with a sheetmetal part and then use blend or boundary blend to generate the wall.

Also, take a look at these posts:

How can I unbend square to ellipse blend in creo sheetmetal? I am using sketched rip.

Modeling Developable Hull Surfaces With Creo 2 .pdf

You can not create a sheetmetal flat pattern from parts like this. Sheetmetal bends must have a single radius (look up press brake bending) You're part has a small radii on one end and a large radii on the other (not a single bend press brake operation).

The link given by @psobejko with the part is a good way to use sheetmetal to get a flat.

Another option is to use surfacing and unbend using the flatten quilt option. I'm not surfacing guy, but it can be used to get the flat.