Cannot delete asset with AssetDatabase.DeleteAsset?

when I try to delete .asset I get this error:

Failed to remove asset meta data because some of its object's were still loaded!

here's the code:

     public void CopyCurrentStorage(string stPath, string path, string name){
     string toPath = path +"/" +name +".asset";
     if(System.IO.File.Exists(Application.dataPath +"/" +toPath))
         Debug.Log(AssetDatabase.DeleteAsset(toPath));
         //if(AssetDatabase.DeleteAsset(toPath))
         //    AssetDatabase.CopyAsset(stPath, toPath);
     else AssetDatabase.CopyAsset(stPath, toPath);
     
     AssetDatabase.Refresh();
 }

How could I 'unload objects' from .meta? (Deleting from editor with pressing [del] works fine)