About the problem of not being able to go to the next level of line MAT

mac os12.5.1
The shortcut keys also do not allow access to the next level of line mat or constant mat
Is there any other way

Could you explain what you mean by “next level of a MAT”? Also which shortcut combination are you referring to?

Want to go to the next level of the material, go inside and operate
Shortcut Keys“enter”and “u”“i”No effect

Maybe a screengrab would help, because materials or MATs are not components so you can not ‘go inside’ them. You can only control their parameters just like other standard OPs (TOPs, SOPs, CHOPs DATs).
Components (COMPs) are the only structure you can go inside.