Actually, the penalty for an excess SEP contribution is 10%, calculated on Form 5330. TurboTax does not support Form 5330.
Most people make their SEP contribution for a particular tax year shortly after the end of the year. Since SEP contributions are reported as to the year in which they are made instead of the year for which they are made, If your made the SEP contribution for 2018 in 2019 you can just treat that contribution as a combination of your maximum permissible SEP contribution for 2018 plus part of your SEP contribution for 2019 and none of it will be an excess contribution. However, if you made an excess contribution for 2018 in 2018, you are subject to the 10% penalty on the excess.
