Serial communication between an Uno and an Esplora by connecting their usb ports together
It will not work. USB communication is master/slave. There must be a 'host' and a 'device'. Uno and Esplora are two 'devices'.

Connect the boards over TTL Serial. Wire Rx and TX pins together, TX to RX and RX to TX. And ground of course.

Serial on Esplora is the USB interface. In 'core' I see the Esplora has a second serial interface called Serial1. According to schematics, the RX and TX pin of Serial1 on Esplora are on LCD header as DC_LCD (RX) and RESET_LCD (TX). Use this Serial1 to communicate with Uno.

On Uno side you should disconnect the Uno from computer to not disturb the Serial interface with USB connection. Or use SoftwareSerial to communicate with Esplora and have debug prints to Serial Monitor.