A Strix class is in a namespace different from C# standard classes or Unity classes, so you need to add an appropriate using in your C# script. Basic STRIX classes such as StrixNetwork or StrixBehaviour are defined in the namespace SoftGear.Strix.Unity.Runtime. You should add the following using in the series of usings at the top of the script:
using SoftGear.Strix.Unity.Runtime;
If the above using line itself causes CS0246, it means Strix Unity SDK is not imported in your project appropriately. Please consult the documentation and try importing Strix Unity SDK again.
Some Strix features may need other usings. If you have written the above using, and names such as StrixNetwork are compiled, but another STRIX name causes an error, please refer to the Strix Unity SDK API Reference to know the namespace that the name you are using belongs and add an appropriate using.