You need to have some available KSMs in your wallet to pay the transaction fee in addition to the bound KSMs, otherwise you will get the error “Invalid Transaction: Inability to pay some fees”.

Please transfer about 0.01 KSM to your wallet, then you can unbind KSM successfully.

Note: There is a 7-day waiting period for the unbound KSMs, after which you can get back your KSMs.