The IRS's database indicates that your dependent's SSN has already been used on a 2019 return, as the primary taxpayer or spouse on that return. Once that's occurred, the IRS won't accept another e-filed return with that SSN, giving rise to the need for a mailed return.
See this TurboTax support FAQ for printing a tax return for mailing - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
