how to save a unreadable texture in runtime

i got the texture from an assetbundle and i can see it in the editor ,use it in run time but how to export it from asset bundle???? i try to use assetDatabse.createAsset ,but it logs an error"Couldn't create asset file because the Texture2D 'effect_101' is already an asset at '' i try to use texture2d.encodetopng(). it also logs an errot "UnityException: Texture 'effect_563' is not readable, the texture memory can not be accessed from scripts. You can make the texture readable in the Texture Import Settings."

i don't have the texture ,i can only get the asset bundle

so ,anyone has a method to export it ????

thankssss