Unable to add reference to installed NuGet package?

I created a NuGet package and I was able to successfully install it in another .NET solution. But I'm not able to add a reference to the NuGet package from the other .NET solution.

For example, the NuGet package has a class with a namespace like MyCorp.SecurityApi. I'm currently not able to add a using directive for that namespace in my other .NET solution. For example, using MyCorp.SecurityApi directive returns this compilation error:

The type or namespace 'MyCorp' could not be found

Any idea what the issue might be or how to debug it?