It is not a Tax ID. It's a tracking ID (some firms use the term Document ID) for ML it's located on its Tax Info statement at the bottom of page 1. You may need to access that statement directly to find the 24-character code. Then go back to TurboTax to enter it.
Some firms use the normal login to access the tax documents to be imported. Others like ML use a different method that requires a document or tracking ID that you won't have unless you got the document through the mail or can access the document through an e-mail link (can't import from this - only for getting the iD.