Hi,

To assign a default expense account, add a default attribute to the corresponding field, and add constraints for tax fields to prevent adding more than one tax to form.

class HrExpense(models.Model):
    _inherit ='hr.expense'
def _default_account_id(self):
    return self.env['account.account'].search([
        ('account_type', 'not in', ('asset_receivable', 'liability_payable', 'asset_cash', 'liability_credit_card')),
        ('company_id', '=', company_id)], limit=1).id

account_id = fields.Many2one( 'account.account',  compute='_compute_account_id',store=True,
    readonly=False,  precompute=True,  string='Account',   default=_default_account_id,
    domain="[('account_type', 'not in', ('asset_receivable', 'liability_payable', 'asset_cash', 'liability_credit_card')), ('company_id', '=', company_id)]",   help="An expense account is expected" )

@api.constrains('tax_ids')
def _check_tax_ids(self):
    for record in self:
        if len(record.tax_ids) > 1:
            raise ValidationError('You can add only one tax.')

Hope it helps,