Flex World is one of the many locations in LEGO Dimensions 2: The Rise of Enoch, from the Flex Seal franchise.
Flex World is an entire realm made of liquid rubber (in a can!). It is a world created by Phil Swift, our Prometheus.
In the closing moments of Flex Tape II: The Flexening, JonTron meets Phil Swift in Flex World after being pulled into the T.V. by his hand. Jon becomes awestruck and asks to drive the buggy from the Flex Glue episode, to which Phil obliges, but then denies after learning that Jon doesn't have his license or insurance papers on-hand. After this, Phil takes Jon into a vault where he shows him something inside of a red toolbox that proves to be so awesome that it grays Jon's beard, eyebrows and the hair along his hairline. He then has Jon sign a stack of legal documents, before sending him home. It was later revealed to be a dream world to JonTron.