Your base currency is chosen when the store is created. It cannot be changed, disabled, or removed. Catalog prices start from it, and reports show totals in it.
To sell in more currencies, your plan must include multi-currency. Turn on Multi-Currency in Settings > Currencies, then add the currencies you want. Customers see an added currency only after you assign it to their country in a market.
Open Settings > Currencies in Store Admin or the mobile app. Staff need permission to view currencies to open the page and permission to manage currencies to change anything. The page shows how many more currencies your store can add.
To turn Multi-Currency off, remove every additional currency first.