"Smart Pricing floor price needs to be between $X and $Y" error is given when your listing that has a smart price cannot be listed due to the update in the smart pricing range. For instance, you posted a listing with a smart price of $50, and Mercari changed the smart pricing range for that listing to $20-40, your listing is not available to be listed with the price of $50 anymore.
How to fix "Smart Pricing floor price needs to be between $X and $Y" error:
You need to edit the smart price of your listing manually in accordance with the new smart pricing range given by Mercari and save your listing. After you update the smart price of your listing, you can use PrimeLister for the next list posting operations.