For scripting questions, please post in the scripting forum. The 2D forum isn’t for general scripting questions. Your question is the most common question asked on this forum and is a basic C# thing; you have a reference that isn’t initialised. There’s a pinned post here: https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/
A good place to start is the Getting Started forum for initial questions too.
Also, please use code-tags when posting code rather than plain-text.
I’ll move your post for you.
Thanks.