The actual reason why you can't do either of the following,

l = [].append(2)
l = [2,3,4].append(1)

is because .append() always returns None as a function return value. .append() is meant to be done in place.

See here for docs on data structures. As a summary, if you want to initialise a value in a list do:

l = [2]

If you want to initialise an empty list to use within a function / operation do something like below:

l = []
for x in range(10):
    value = a_function_or_operation()
    l.append(value)

Finally, if you really want to do an evaluation like l = [2,3,4].append(), use the + operator like:

l1 = []+[2]
l2 = [2,3,4] + [2]
print l1, l2
>>> [2] [2, 3, 4, 2]   

This is generally how you initialise lists.

your look will only iterate if there are items in the list, so just take the first if outside the loop to check if the list is empty or not

# Task 1a
# Create a function that places an element x in a sorted list

def insert_in_sorted(x, sorted_list):
    if not sorted_list:
        sorted_list.append(x)
        return sorted_list
    
    for i in range(len(sorted_list)):
        if x > max(sorted_list) or not sorted_list:
            sorted_list.append(x)
            return sorted_list
        elif sorted_list[i] > x:
            sorted_list.insert(i, x)
            return sorted_list
        elif sorted_list[i - 1] and sorted_list[i + 1] == x:
            sorted_list.insert(i, x)
            return sorted_list


print(insert_in_sorted(2, []))
print(insert_in_sorted(5, [0, 1, 3, 4]))
print(insert_in_sorted(2, [0, 1, 2, 3, 4]))
print(insert_in_sorted(2, [2, 2]))

OUTPUT

[2]
[0, 1, 3, 4, 5]
[0, 1, 2, 2, 3, 4]
[2, 2, 2]

Do it this way

''' You dont need to send pp and passengerInput as parameters for this function because, they're values are being initialized only when the passenger_list() is called. ''' 
def passenger_list(): 
    ''' It would be better for you to avoid having the passengers text inside. If you really want the values as passenger: "name of passenger", you can use a dictionary. I will add code for that as well.'''
    pp = []
    passengerInput = input("what is your passanger name?")   
    if passengerInput:
        pp.append(passengerInput)
    print(pp)
    
    # This statement is crucial so that the value is passed back to the code that is calling it.
    return pp

In this case, after calling passenger_list(), your output will look like ["Ram", "Shyam", "Sita"].

For output of the form - {"Passengers":["Ram","Shyam","Sita]}, please refer the code below.

def passenger_list(): 
    
    pp = {"Passengers":[]}
    passengerInput = input("what is your passanger name?")   
    if passengerInput:
        pp["Passengers"].append(passengerInput)
    print(pp)
    # You can access the list using pp["Passenggers"]
    print(pp["Passengers"]) # Output is ["Ram", "Shyam", "Sita"]  
    
    return pp 

Read more about Python dictionaries here.