You can put a Total Contribution of $26,000 in 401K if you are over 50, so this is not an error.
$19,500 for a regular contribution, plus $6,500 'catch up' contribution.
The 'catch up' has to do with your age, not how many years you are with the company, so unclear what the $3,000 'catch up' you are referring to is.
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Do you have an amount on W2 box 12 for more? Then check with your employer. They may have let you contribute too much. Or are you trying to enter this in the IRA section? It is not an IRA. You only enter it from your W2 nowhere else. It has already been deducted from the Wages in box 1 and you can't deduct it again.
This is apparently a 403(b) with a special $3,000 catch-up. I don't see that TurboTax's Smart Check erroneously flags this as an error in the CD/download version of TurboTax, but if in the final check before e-filing TurboTax erroneously says that your total of $29,000 of elective deferrals to the 403(b) include an excess contribution because they exceed the $19,500 regular limit plus the age-50 $6,000 catch-up and prevents you from e-filing, you'll have to print and mail your tax return.
Note that if you contribute to any other 403(b), 401(k) or federal TSP plans, your total elective deferrals to all of these types of plans is not permitted to exceed $29,000 (including your $3,000 special catch-up contribution to the 403(b)).
There are two catch-up options and both can be used. The over 50 which most people know about, and 15 years with the same company catch up. So a profession like a teacher, can do $29,000 for 2020 - $19,500, the $6,500 catch up, and the $3,000 same company catch up.