It is because you are in the assembly environment. In assembly, you can only made cuts with the extrude feature.
If you need to extrude a new feature, you will need to enter the part where you want that new feature (by either opening it, or edit in context). Or create a new part for the new feature and then place it in your assembly.
Aaron Jarrett, PEInventor 2019 | i7-6700K 64GB NVidia M4000
LinkedInLife is Good.