You might want to add Assets from the Unity Assets Store to your project (see ). This often ends up in having some errors, reflecting failures of importing, dependencies or others. The following explanation should give you a clue about the sequence to install packages and a way, how to deal and/or fix with errors.
First ignore the yellow alerts in the console, errors are always shown in red.
Assuming, you have installed Unity3D and created a new project without installing Standard Unity Packages. Many package creators pack standard assets into their packages and distribute it. This can cause errors if objects of the package are outdated or simply just installed by another package. Therefore please do not install any of the standard packages.
1. On a fresh unity installation create a project
2. Deselect all standard packages
3. Install Sine Editor Package (actual version)
The console will come up with the below shown error. Don't think about that - this error can simply be ignored because it's only saying, that Sine has already packed the InputManager/GraphicsSettings assets.
4. Assuming, you want to install GAIA (Terrain Tool, which can be purchased in the Unity Asset store). Switch to the asset store, and download it
5. import GAIA
6. Now, the console window is showing additional errors. Note: always read the documentation of the package creator. In case of GAIA it clearly says, that the material for the TreeObjects has to be rebuild.
7. Simply follow the description of GAIA and rebuild the material for all 4 Tree objects (every object which is rebuild, reduces the errors by removing the corresponding line.)
8. If you are ready, all errors shown by the console window, should have disappeared except the one described in topic 3. You can also tell the system, to remove this line by just clicking 'clear' in the console window.
If you want to install Asset Packages, it's better not to install the Standard Assets - it's better, to install all required (add on) packages first and than add the required Standard Packages if needed. All Items, which are just installed by the add-on's, will be excluded during the import of Standard Packages.