How level does the cement floor need to be for this to be installed?

Hello Lisa and thanks for your question. When installing the Style Selections Rigid Core Vinyl Plank on a concrete or plywood substrate, the subfloor does not have to be perfect, but it should be flat, with a maximum acceptable tolerance of 1/8" on a 6 foot span. Any gaps or voids need to be filled with a commercial grade cementitious patch compound to avoid deflection (bouncing) or hollow pockets. This is really to prevent the locking mechanisms from breaking or coming apart at a later date. All subfloor prep can be found in the Installation Instructions (located in each carton or on the product page of Lowe's website). Thanks again.