The error message 'prefab unity3d does not exist' is typically an outcome of one of the following situations:
Invalid Path or Name: You are trying to access a Prefab that Unity can't locate because the path is incorrect or the file does not exist in the specified directory. This often happens if you've moved or deleted the Prefab file and haven't updated the reference in your script.
Incorrect Spelling or Case Sensitivity: Unity is case sensitive. If there is any mismatch in the casing or spelling, it will not be able to find the prefab.
Prefab Not Loaded into Memory: If the Prefab is not loaded into memory at runtime, perhaps because it is not attached to a GameObject in the scene, or if it's not included in the 'Resources' folder, Unity will not be able to find it when trying to access via a script.
To resolve this issue, take the following steps:
Check File Existence and Path: Ensure that the Prefab file exists where you expect it to be. Confirm the path to your Prefab and make sure it matches exactly with the one in your script.
Correct Spelling and Case: Verify the spelling and case sensitivity of your Prefab name both in the script and in the Project window. They should match exactly.
Load Prefab into Memory: If a Prefab isn't used in the scene (i.e., not attached to a GameObject), it might not be loaded at runtime. One way to solve this is by placing the Prefab into a 'Resources' folder. You can access it in your script using
Check for Compilation Errors: Ensure there are no script compilation errors which could prevent Unity from recognizing and utilizing your Prefab correctly.
By ensuring the correct spelling, case sensitivity, file location, and appropriate loading of prefabs, you should be able to eliminate the 'prefab unity3d does not exist' error.