Currency fluctuates depending on supply and demand

In fixed exchange rate, any central banks could ties their currency to another country's currency or Gold price. Which, basically, still fluctuates

In what scenario would it be possible to create currency that is **truly fixed**? For example, create GGG currency that ties 1 USD at today's exchange rate

1 GGG = 1 USD at 9/28/2021

Means at any time onwards, will still be using the same exchange rate