I believe there are two issues here.
Only certain version of .netstandard work with certain versions of .net 4.X.Y
To resolve this you may need to change the version of one or both projects. Use the chart to figure out which versions are compatible together: https://learn.microsoft.com/en-us/dotnet/standard/net-standard
- Select your project
- Right click for properties
- On the "Application" tab use the "Target Framework" drop-down to change framework as needed
.net 4.X.Y uses the 4.0.0.0 system libraries, whereas .netstandardX.Y uses 4.3.0.0
To resolve this you need to add and reference the netstandard.library NuGet package.
- Select your project
- Right click for "Manage NuGet Packages..."
- On the "Browse" tab search for "netstandard.library"
- Select the version you need and install it
(I believe it's based on what version of .netstandard you ended up using)
- Navigate to each reference error, right click on it, and add whatever reference is missing