If the .NET OPC UA application cannot be licensed as described in Licensing the SDK, check the following steps:

Do You Use the Correct Assemblies?

Check the Product Version of UnifiedAutomation.UaBase.dll in the \bin folder of your application. If the product version contains the string "_EVAL", then you are not using the correct assemblies. Use the Release Version of the assemblies instead.
You can also try cleaning the Nuget cache, as it may not update cleanly. By default the cache is located at C:\Users\<username>\.nuget\packages.

Do You Use the Correct License File?

Check if the license file used in your application is binary equal to the license file you retrieved from your download share (WebDAV or ftp).

Note: You should always use the license file that comes with your SDK version!

You can also find the license version in the license file under <ProductId>.
But do not save this file. The content can be changed by the editor and thus damage the key!

Read the Trace file

Look in the trace file for the information FindLicense Failed, which obviously indicates a license problem.
Check that the version information of the Stack Assemlby and License info lines match.

Do You Invoke the Commands in the Correct Order?

ApplicationLicenseManager.AddProcessLicenses must be called before ApplicationInstance.Start is called.

Do You Use the Correct License Type?

If the OPC UA application is a server application, you need a UaSdkNetBundle license.