how to flat pattern in inventor from imported STEP file ?

You can’t produce a flat pattern directly from that part. That’s an extruded “Z” shape. It has varying thickness across the profile. This part was not designed to be made in a flat profile.

However, if you round the outside corners with a 10mm radius so that it has a uniform cross section then a flat pattern may be obtained:

1. Set your material in the physical properties.

2. Round the outside corners with a 10mm fillet.

3. Set your sheet metal defaults to have a 4mm thickness.

4. Define A-Side and select one of the flat surfaces.

5. Go to Flat Pattern.

Note: It may not be a usable flat pattern. Many features cross the profile at odd angles. Inventor will faithfully reproduce these features wrapped into the flat pattern. You couldn’t simply cut a profile using this flat pattern.

Attached is an Inventor 2022 file with the flat pattern.