Np tak:
import requests
url_template = "http://api.nbp.pl/api/exchangerates/rates/a/{currency_code}"
while True:
value, currency_code = input("Podaj kwotę do przeliczenia. Np 12.5 CHF\n").split()
rate = requests.get(url_template.format(currency_code)).json()["rates"][0]["mid"]
print(f"{value} {currency_code} = {rate * float(value)} PLN")