The inaccessible coins are on the tile tinted green.
In Pokémon Red, Blue, and Yellow, the inaccessible coins are a batch of 10 hidden coins in the data of the Celadon Game corner that cannot be picked up. They are located on the same tile as one of the slot machines (which is itself normally unplayable, due to an NPC occupying it). Presumably the developers intended it to be able to be picked up from the tile below, but due to how the game handles interactions with slot machines and hidden coins, this wouldn't work.
Internally, both slot machines and hidden coins are regarded as "hidden objects". This means that, in the hidden objects list of the Celadon Game Corner, there are two entries (here and here) with the same coordinate (15, 12). When the player tries to interact with this coordinate, only the first entry, the slot machine, will be found. Even though interacting with a slot machine from below does nothing, this will still prevent the player from picking up the coins.