Please help me solve this.i want to make the mainmenu appear only after succesful sign up and if the username and password is wrong back to main page

Please help me solve this.I want to make the mainmenu appear only after succesful sign up and if the username and password is wrong back to main page

What I have tried:

<pre>print("\n Welcome to Covid 19 Vaccination App ")
def SignUp_login(welcome):
    if welcome == "1":
        name=input('\nEnter Your Name: ')
        username  = input("\nEnter a username:")
        phone=input('\nEnter Your Phone number: ')
        password  = input("\nEnter a password:")
        password1 = input("\nEnter a Confirm password:")
        if password == password1:
            try:
                file = open(username+".txt", "r")
                data   = file.readline().split(':')
                if username in data[1] and password in data[-1]:
                    print("\nUser already found")
                    mainmenu
                
            except:
                file = open(username+".txt", "w")
                file.write(name+':'+username+':'+phone+":"+password)
                file.close()
                print('\nYou Successfully SignUp !')
                mainmenu
        else: print("\nPasswords do NOT match !")
        SignUp_login
    elif welcome == "2":
        login1 = input("\nUserName : ")
        login2 = input("\nPassWord : ")
        try:
            file = open(login1+".txt", "r")
            data   = file.readline().split(':')
            if login1 in data[1] and login2 in data[-1]:
                print("\nWelcome")       
        except:
            print('\nUser not found please Sign Up!')
            SignUp_login if FileNotFoundError else SignUp_login
welcome = input("\n1. SignUp \n2. Login \n\n")
SignUp_login(welcome)

def mainmenu():
   print("What would you like to do?")
print("1. View Medical History and Status")
print("2. Update Medical History")
print("3. Check Vaccination Appoinment(Date,Time,Location)")
print("4. Quit")
selection=int(input("PLease Enter a choice: "))
def Status():
    print('Your Status is currently')
def Fill():
    print('l')
def Check():
    print('Your Appoiment Is Currently at')
if selection==1:
    Status()
elif selection==2:
    Fill()
elif selection==3:
    Check()
elif selection==4:
    exit()  
else: print("Invalid choice.Please Enter A Valid Choice from 1 - 4")