How to make this hole through this important object

I am trying to make 6 circular holes in the side of the box but when trying to use Extrude on this area of the box (the middle section), the remove option is gone and the face of the box can't be changed, is there a way to fix this or a solution. I have provided screenshots to this issue and would appreciate any assistance.

This is the link: https://cad.onshape.com/documents/e30b3626202e1559e6ef9d17/w/905974c6502c8822901f3865/e/8827ef9ec4bf4b2326489256?renderMode=0&uiState=66e09a940a01417638bbedbe

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