If you are able to log in to the Pokémon HOME app without issue, or if the Nintendo Account login screen does not appear in the Pokémon GO app, there may be an issue with the Pokémon GO app itself.
Please visit the Niantic, Inc. support site and check the "Contact Us" page for instructions on how to contact Niantic, Inc., the developer and operator of Pokémon GO.
■Pokémon GO Support Site
https://niantic.helpshift.com/hc/en/6-pokemon-go/