The only way to enter Saffron City is by going through the gates into the city, one of which is located in Cerulean City, but getting through is impossible at first. All guards on duty at the gates are thirsty and won't let any players through. The only way to quench their thirst is by giving them tea.
Where to get tea in Pokemon Let's Go
At that stage in the game, it's impossible to get the tea without furthering the progress in-game first. Use the Underground Path outside of Cerulean City that connects Route 5 and Route 6. Players will end up in Vermillion City on the other side and will continue progressing through the game in Vermillion City.
Step 2: Go to Lavender Town through the Rock Tunnel
After completing everything in Vermillion City, players will need to use the Underground Path to get back to Cerulean City. Once in Cerulean City, players will continue east on Route 10 and eventually see the Rock Tunnel. When players come up from the other side, they will arrive in Lavender Town where they will need to continue their progress there.
Step 3: Use Underground Path to Celadon City
The second Underground Path connects Route 8 and Route 7 together, bringing players to Celadon City. After entering Celadon City, players must continue walking to the left, past a man with a Poliwhirl, and will find Brock standing outside of the Celadon Condominiums. He will talk with the player for a bit before leaving the player with Pewter Crunchies and tea.
Step 4: Give the tea to any guard
Any of the four guards that are outside of Saffron City will stop players if they try to walk past, but will notice the newly obtained tea. After letting the guard drink the tea, he will allow the player to continue to Saffron City and will share the tea with the rest of the guards -- which will free up the entrance to Saffron City from every other gate.
Players can continue on to Saffron City to get their sixth gym badge from Sabrina.
Cover photo courtesy of Nintendo