* Did you check the right people? The three assistants who teach you HMs are located in these spots:
* Goldenrod City: The assistant in the Goldenrod Game Corner, who teaches Cut.
* Cianwood City: The assistant in the Gym, who teaches Strength.
* Azalea Town: The assistant at the Azalea Town Gym, who teaches Surf. Make sure you've spoken to all three of them.
* Did you complete all three regions? You mentioned beating the Johto and Kanto Elite Four, but did you fully explore all three regions (Johto, Kanto, and the Sevii Islands)? There are some trainers in later regions that might have Flash.
* Did you use a cheat or glitch? Sometimes, cheats or glitches can interfere with the normal game progression, including HM acquisition. If you've used any cheats or experienced glitches, that might be the reason Flash isn't available.
* Did you trade for a Pokemon already knowing Flash? If you traded with someone for a Pokemon that already knew Flash, it should be available to learn without having to get it from an HM.
To troubleshoot further:
* Check your Pokedex: Make sure you've caught all the Pokemon in the game to ensure you haven't missed a trainer who might give you Flash.
* Try restarting the game: Sometimes restarting the game can fix glitches or refresh the game's logic.
* Search online for a specific guide: Look for a guide for Pokemon Gold that specifically details how to obtain Flash. There might be some hidden detail or requirement you've missed.
Good luck finding Flash!