The inability to withdraw BTC to your Avacus wallet is due to the exchange's security features functioning correctly.
The addresses generated by the Avacus app start with "0x" and do not support receiving Bitcoin (BTC).
If these security features were not in place and BTC was mistakenly sent to an Avacus address, the assets would become irretrievable and permanently lost.
The Difference Between the BTC Network and Ethereum Network
BTC cannot be received directly by Ethereum-based (EVM) wallets. While wrapped tokens like WBTC (tokens whose value is pegged to BTC) can be received, they are only available on a limited number of exchanges and require specifying a network like "Ethereum (ERC-20)" during withdrawal.
Please also note that WBTC is not the same as actual BTC. If you send WBTC to an exchange that only handles BTC, those assets will also be lost.
When managing or transferring BTC, always use a dedicated app or wallet that supports the BTC network.