Could not resolve this reference. Could not locate the assembly "Xamarin.iOS".

I checked out the perfectly working xamarin forms project to my mac and tried building from VS mac. But I get the below error:
" /Applications/Visual Studio.app/Contents/MonoBundle/MSBuild/Current/bin/Microsoft.Common.CurrentVersion.targets(5,5): Warning MSB3245: Could not resolve this reference. Could not locate the assembly "Xamarin.iOS". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. (MSB3245) (Assette) "
I tried deleting bin and obj folders, tried unloading and reloading projects but the issue still exists. But when I create a new Xamarin.Forms project from VS it perfectly builds and runs without any error. How can I fix this?