For 2019 the 401K contribution max is $19.5K for those under 50-something. I contributed just under the max, but the app is telling me I over contributed, why?

No, the limit for 2019 is $19,000.  For 2020 the limit is $19,500.

Thanks - i still contributed just under 19K, so i'm not sure why Turbo Tax is telling me I over contributed

Make sure to enter items shown in box 12 of a W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax.  Do not mistakenly enter your 401(k) contributions as IRA contributions; a 401(k) is not an IRA.  (I mention this because many people make this mistake.)

If you have participated in plans of multiple employers, the total of your elective deferrals and designated Roth contributions to the plans is not permitted to exceed $19,000.