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Cut 45 degree cut on sheet metal part

in General
Comments
If you share a public model and/or some screenshots it would be a lot easier for people to provide assistance.
Are you aware the the flat view will allow sketch and extrude/remove?
BTW. It looks like you intended to add a link but all I see is a caution icon.
https://cad.onshape.com/documents/e7deb849eaf79cbe21dd8d77/w/d33b9654867f8aafb9a49fde/e/fbf0b581d0f728c6aee041b3
Hint, try the Learning Centre
https://learn.onshape.com/