No, you should not manually correct your Box 12 entries on your W2. You have taken the right steps to include your 401(K) contribution on your 2018 Tax Return.
At this point, you are running into an e-filing issue within TurboTax. An update to fix the issue is expected to be released around 4/5/2019. You can check out this TurboTax FAQ for more information: Excess 401(k) Contribution Preventing E-filing.
Be sure to sign up for updates to be alerted as soon as there is a solution. If you do not want to wait for an update, you have the option to paper file your tax returns as this is only an e-filing issue.
