I want to combine materials

Please have a look at this movie.
I clicked the right mouse button to conmine two materials.
But the aicon(Texture conbination) was not on the context menu.
Please tell me where it is and what should I do?

This image tells you the problem.
One part of your selection is a group, it only works with raw geometry in the same context.
Also you should be working on the white faces, they are the default front faces.

Can you be more specific?
The my translation system does not function well.
I do not know what mistake I made.
I apologize for any confusion.

The faces you want to combine must not be part of a group or component.
Your video shows a group in your selection, explode it.

One material must be a ‘texture’ not just a ‘colour’.

The context menu is not on the movie,but I did.
I took the group apart.
Am I on the right track?

I think you are trying too hard.
No grouping needed.

if you only want to use colors, you need to make one of them ‘Unique’ in the context menu, first.
Than select all and choose ‘Combine textures’

I can not center a surface on another surface if I made a grope.

There is not command in the first place.

I don’t know how to explain to you that the faces all need to be together.
In the first image you can see that the 6 faces have the blue selection dots, but the rest of it doesn’t, suggesting that the outer face is a separate group. You can also see z-fighting, but I have no idea how that will translate.

This video from SketchUp gives you more insight into how to combine textures.

I see how it is.
I tried to take apart first and second grope from the bottom.
Blue dots were displayed.
Both of you, Thank you for teaching me so patiently.