What is the biblical source for not being able to toivel a Kli Cheres?
It is not that we can’t tovel it, rather it is that it is not required and it may be used even if it was purchased from a gentile without the need to tovel it. The torah tells us (Bamidbar 31-22) that any metal utensils that the Jews got from their war with Midyan had to be kashered, and then placed in a mikvah (see Tractate Avoda Zara 75b). Chazal learned from this, that after kashering them they have to be toveled. Technically this applies only to metal utensils and nothing else. Chazal however said that glass also needs to be toveled, since it has properties of metal, i.e. that it can be melted and reshaped. Therefore the only materials that need toveling are metal and glass, and earthenware vessels were just never included in this mitzva.