I cannot receive BTC from my source because they say my Bisc wallet address is invalid, because it starts with B and not 1.
Bitcoin addresses can start with 1, 3 or bc1, but not all wallet software support all address formats. Especially bc1 (=Segwit) is not always supported.
What do you mean by source? Is that a third party that should send you bitcoins to your Bisq wallet?
You may generate a new address starting with 1 in Bisq when uncheck “Native segwit format” and clicking “Generate new Address” in the Tab Funds>Receive Funds.