1) You know, that you do not have to extract parts; you can use linked parts instead - right?

2) Before you can extract a part, the corresponding linked part must be created - since Finale creates the extracted part from the corresponding linked part.

To create the linked part, go to

Document menu > Manage Parts…

3) When the linked part has been created, go to

File menu > Extract Parts…

I still don't understand. When I want to create any other part, I go to Extract parts in the File menu and then the part comes out on my screen However with the piano part, there are two staves and I can only press one of them at a time. How do I extract  both staves of the piano part.

Is there a reason why this won't work?

I don't want to create a piano reduction, I want to extract the piano part from an orchestral score. The piano part is on 2 staves. I can click on only one at a time.

…  I still don't understand. When I want to create any other part, I go to Extract parts in the File menu and then the part comes out on my screen However with the piano part, there are two staves and I can only press one of them at a time. How do I extract  both staves of the piano part …

Perhaps a linked part (for Piano) has not been created?

Take a look at

Document menu > Edit Part > …

In the sub-menu Edit Part, is there a (linked) Piano part?

If the answer is yes, then what does that linked part look like? (one or two staves)

(to view the linked Piano part, go to Document menu > Edit Part > Piano)

If a linked part has not been created, follow step 2) in my previous post.

Can you post an image of your Score Manager window?