If you do have taxes owed then you should file today, pay the taxes owed and then file an amended return after you receive all the information you need.
If you do not e-file the 2018 tax return today, then the 2018 tax return can only be printed and mailed at a later date. The TurboTax online editions for 2018 will be shutting done to make way for the 2019 online editions.
You will need to use the TurboTax desktop editions for 2018 to complete the tax return. You can download the tax data file from the online edition to your computer to save your work.
See this TurboTax support FAQ for saving the online tax data file - https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-file/01/25740
If you will not be owing any taxes on the 2018 return or you are receiving a tax refund then there are no penalties for filing after the due date.