She needs to amend and check the box that she can be claimed as a dependent.  Amended returns take about 4 months to process.

That will not allow you to e-file this year.  You only option is to print and mail file.

You will not be able to e-file your own return for 2018 no matter what.  You have to print, sign and mail your 2018 tax return, since your granddaughter's SSN is already in the system now.  SHE has to prepare an amended return and print, sign and mail it to the IRS to change her answer to the question about whether she can be claimed as someone else's dependent.  It will take about 4 months for the IRS to process her amended return.  YOU do not have to wait for that--just go ahead and mail your own return that claims her.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

For your dependent:

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed?...

A 1040X cannot be e-filed.  It must be printed and mailed.  It will take about 3-4 months for the IRS to process an amended return.  It is a good idea to use a mailing service that will track it, like UPS or certified mail, so you will know it was received.

Wait about 3 weeks from mailing and you can check the status of your amended return:

https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1

https://www.irs.gov/filing/individuals/amended-returns-form-1040-x/amended-return-frequently-asked-q...

https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend

https://ttlc.intuit.com/questions/2566895-what-does-it-mean-to-amend-a-return

If it is any comfort to you, this is a very common problem--teenagers and students often do this without realizing the problem they will cause for the person who can claim them as a dependent.  Then they learn the hard way.

Your granddaughter would need to amend her tax return and change her dependency on the tax return.  Amended tax returns, Form 1040X, can only be printed and mailed, they cannot be e-filed.  The IRS will take up to 4 months to process the amended tax return.

You can complete and file your tax return with her entered as a dependent but you can only print and mail the tax return since her tax return has already been filed and accepted by the IRS with her SS number.

See this TurboTax support FAQ for the procedure to print and mail a tax return when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...