Cant add script

New to unity and visual studios currently using Unity 2019.3.4f1 and visual studios 2019. So when I downloaded an asset package none of the scripts in the package could be added to any of the object in the Hierarchy. Also when attempting to create a new script from within Unity and attaching it to say a display panel for example it says there are compile errors although it's a brand new script auto generated with the start and update methods only. Visual studios also will not recognize much of the syntax from Unity I has tried adding using Unityengine.ui and it does nothing. So I'm at a loss any help would be appreciated.