1. You are attempting to write to an original cartridge
If you are trying to write game data on an official cartridge, the option will be grayed out and unavailable.
As a direct result of their design, original games are stored on ROM memory (read-only memory) which means that no game data can be written to it, the existing data can only be read.
These official cartridges have data burned onto them at the factory and the operation is permanent.
2. The flashcart is currently unsupported
If you connect a known flashcart/rewritable cart and the option is grayed out, it is possible that we do not currently support it. In this case, please send us an email to firstname.lastname@example.org and we'll make sure to add it to our list.