A Credit Payment Calculator
The one thing that is immediately apparent to me is that parsing of command line arguments could be done better using the built-in argparse module but for an intro you can start with the tutorial.

Here is how the routine could be written, assuming all parameters are mandatory:

import argparse

# check command line options
parser = argparse.ArgumentParser()

parser.add_argument("--type", dest="type_of_payment", type=str, choices=["diff", "annuity"], required=True, help="Add some help text here")
parser.add_argument("--periods", dest="period_count", type=int, required=True)
parser.add_argument("--payment", dest="monthly_payment", type=float, required=True)
parser.add_argument("--interest", dest="credit_interest", type=float, required=True)
parser.add_argument("--principal", dest="credit_principal", type=float, required=True)

args = parser.parse_args()

# show the values
print(f"Type of payment: {args.type_of_payment}")
print(f"Periods: {args.period_count}")
print(f"Payment: {args.monthly_payment}")
print(f"Interest: {args.credit_interest}")
print(f"Principal: {args.credit_principal}")


This is more flexible, because:


you can provide parameters in any order
you can assign default values
you can specify the expected type and also restrict to a specific range of values - in this example type_of_payment must be either "diff" or "annuity"
you can also define your own function if you require more fine-tuned validation of certain parameters
you can easily define groups of mutually exclusive parameters


This is an example but you can customize it further. Then your code will become quite shorter = less distraction and more focus on the important stuff.