Last name is more than 20 characters. how to enter the last name?

[sivaemi49]

Will there be a problem or audit if the name is truncated or does not match the official documents?

[DoninGA]

The IRS compares the first four characters of your last name to the Social Security number entered on your tax return.  So you should be fine to truncate the last name on the tax return.

[TomK2023]

No.  Enter the first twenty characters.  That information, plus your Social Security number, will be sufficient to allow the IRS to verify your identity and locate your income tax records.