It is important to make sure that the Bank Account has been set up as a General Ledger Account code in Xero.
To check this in Xero, open the account under Accounting > Chart of Accounts. Check to see if the Bank Account has an Account Code to the left of the Account Name.
Tradify needs the Account Code to reference the account when posting payments entered on an Invoice.
Click on the Bank Account name to edit the account and enter a General Ledger Account Code in the relevant field.
Then go back to Tradify and go to Settings > Accounting System. You will now be able to select the Bank Account in the appropriate field.