AttributeError: 'str' object has no attribute sleep

In this line: time = datetime.datetime.today().strftime("%d.%m.%Y %H:%M:%S")

You are overwritting the time variable, that contained a module and making it a string.

This is an example of working code

#!/usr/bin/python3

import os
import json
import datetime
import cgi
import time

def save(number_input, current_time):
    i = 0
    while os.path.exists("datei/datei{}.txt".format(i)):
        i += 1

    datei = {
        "input": number_input,
        "zeit": current_time
    }

    with open("datei/datei{}.txt".format(i), "w+") as file:
        json.dump(datei, file)

form = cgi.FieldStorage(encoding="utf-8")

number = form.getvalue("first")
time_str = datetime.datetime.today().strftime("%d.%m.%Y %H:%M:%S")

save(number, time)

print("<p>Sie haben {} in einer .txt Datei gespeichert!              </p>".format(number))                              
time.sleep(4)
print("Location: main.py")
print()

Notice I've changed time to time_str, this way your time variable is untouched and you can call sleep :)