I cannot see my clearing account in the Prepayment account list.
For prepayments, the clearing account must be a bank account in Xero. The bank account does not have to be a real bank account (Account number and BSB can be incorrect) to be used. When your settlement appears in your bank feed (to your real bank account), it will get reconciled to this prepayment clearing account (fake bank account) as well as the clearing account used for invoice payments.
Can Invoice payments and Prepayments use the same clearing account?
Yes, but the account will need to be a bank account type in Xero. Your bank account in Xero does not have to be connected to a real bank account (Account number and BSB can be incorrect) to be used.