I have tried to correct an over contribution to a 401k following the instructions from Turbo Tax, but it still says it must be fixed when I go through the final check.

At this point, I think 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.

If you haven't done so already, use the steps in this answer to include your excess contribution as wages on your 2018 Tax Return:  What to do if you have an excess contribution to a 401(k).

Has the update been released?  I am still having an issue.  I am using the online version of Turbo Tax.

I'm also confused why it recommends code P.  If I am trying to correct an over contribution made in 2018 and I am working on 2018 taxes wouldn't I want code 8?  

I've followed all of the steps and it still says that I have a 401k over contribution.  I would really want to avoid paper filing.  Why is it saying that I still have an over contribution?