What would cause a palpable and visible lump in neck to not show up on ct scan? About the size of grape...

Possible lipoma. Regardless of normal imaging, any palpable finding should be evaluated and followed by your PCP. However, lipomas just below the skin surface are common and frequently on imaging do not show a capsule or margin and just blend in with the surrounding fat. Contrast enhanced MRIs are frequently obtained to further evaluate and confirm such a suspicion.
It got ignored. Unless your physician notes that the lump is in question, a radiologist won't report a benign-appearing neck mass like this. Grape-sized lymph nodes are very common, especially in otherwise-healthy young folks like I'll bet you are.