For 2018,I am over 50 and I contributed the maximum to my 401k which is 24,500. Turbo tax keeps saying that I Over contributed. How can I correct this ?

You only enter it from the W2, nowhere else.  Box 12 code D.  Did you also enter it under Deductions & Credits?  That is only for IRA contributions not 401K.  Delete it from Deductions & Credits.

Check your date of birth in the MY INFO tab ... make sure the W2 is in your name if you are married ... and make sure you are not entering the 401K contribution in the IRA section.

THANK YOU!! I was entering it in the IRA section. UGH!

YES...THANK YOU!! I was entering it in the D & C section.  

FYI ... if you ever enter something TWICE in the program then you are 100% doing something wrong and the program will call it out for sure 😉

The 401K comes out of your checks pretax meaning before taxes and has already been deducted from the wages in box 1.  So you can't deduct it again under Deductions & Credits.