It previously had sheet vinyl and still has some adhesive residue on it. How do I cover up the residue? I can't get it completely off. Thanks!
The vinyl planks will need a hard smooth surface to lay over. Typically you would scrape the adhesive clean off then use floor patch and self leveling underlayment as required to get the floor smooth and even.
A a wood based laminate floor your could float right over the adhesive because they all typically have or require a thin foam like underlayment that could float right over the old adhesive as long as you got off the peaks and filled the dips in the over floor.
I have found that serious application of a CARBIDE "paint scraper" will remove some truly miserable adhesive residue in a purely mechanical fashion. Don't even bother with the steel blade types, they proved useless.
Typically have a narrow tungsten carbide blade that is clamped, and has two sides so you can flip it once for a fresh edge before replacing or resharpening. Not even terribly expensive.
Get as much as you can off. Go over it a level off the low spots . Add a 1/4 inch underlayment plywood such as louan . Then your pad for floor , Get a good pad mine was of poor thickness and will never use again..And fill all areas !/8 , and a good pad will take care of the rest.