The notice from TurboTax should explain why an amended 2018 tax return is needed. If you can provide that explanation, someone may be better able to help you.
Examples of reasons that TurboTax sends notices of the need to amend are 2018 tax law changes made in 2019 or 2020 that were made "retroactive"; and issues with TurboTax that caused an incorrect calculation of 2018 income tax. When TurboTax identifies customers who are affected or could be affected by a particular issue, it sends a notice to those affected customers.