Rather than these forums being your “fix one error and read back the next error” service, you can actually reason about errors and solve them all by yourself… It’s amazing!
In this case it sounds like you’re using something that is not part of Unity. Perhaps you need to get that BigDouble from some library? Google might be useful here.
How to understand compiler and other errors and even fix them yourself:
https://discussions.unity.com/t/824586/8
If you post a code snippet, ALWAYS USE CODE TAGS:
How to use code tags: https://discussions.unity.com/t/481379
How to report your problem productively in the Unity3D forums: