You indicated that you are using TurboTax Online. With TurboTax Online you have to use a separate user ID for each return. You cannot prepare another tax return with the same user ID that you used for your own return.
See the FAQ at the following link for instructions to start another tax return with a new user ID.
You cannot use TurboTax to do tax returns for other people if they are paying you to do it.
Not if you are a paid tax preparer.
Online Vs Desktop
Online version: With the online version of TurboTax (any flavor) you can only create one tax return per online account. But you can create up to 5 accounts per e-mail address. With the online version you will $PAY$ for each tederal tax return that you "print and file" or e-file. You will also pay a purchase price for each state tax return program, and then you will pay again to file that state return, regardless of weather you elect to "print and file" or e-file the state return.
Desktop version: With desktop you purchase the FEDERAL program one time and install the program on your computer. Then you can create as many federal tax returns as desired. But per IRS limits, you can only e-file a maximum of 5 federal returns. After the 5th federal return, you can print, sign and mail any additional federal returns. You DO NOT pay to e-file federal returns, or to print and file federal returns.
For state taxes, you have to purchase each state program separately and install the program as an add-on to the federal program. Then you will pay again to e-file the state return. You can avoid the cost of e-filing a state return if you instead elect to print and file the state return. With the desktop version only, there is no additional charge to print and file the state return via mail. There is only an additional charge if you elect to e-file the state return.