You might try something like
def validate_user_input():
success = True
try:
pass_credits = int(input("\nPlease enter your credits at pass: "))
defer_credits = int(input("Please enter your credits at defer: "))
fail_credits = int(input("Please enter your credits at fail: "))
except ValueError:
print("Integer Required \n")
success = False
else:
if pass_credits + defer_credits + fail_credits != 120:
print("Total incorrect \n")
success = False
elif (pass_credits not in range(0,121,20)) or (defer_credits not in range(0,121,20)) or (fail_credits not in range (0,121,20)):
print("Out of range")
success = False
return success, pass_credits, defer_credits, fail_credits
success = False
while not success:
success, pass_credits, defer_credits, fail_credits = validate_user_input()