import tkinter
import random
import time
from tkinter import *
r = tkinter.Tk()
tk = Tk()
f = Frame(tk)
r.title("userinterface")
r.minsize(width=400, height=400)
# r.configure(background='gray49')

def viderfunction():
    navnlab.pack_forget()



# def b(Event):
#     def destroy():
#         f.destroy()

b = Button(r, text="vider", command=viderfunction, compound=CENTER)
b.place(relx=0.5, rely=0.5, anchor=CENTER)
b.pack()
navnlab = Label(r, text="navn?").grid(row=1)

alderlab = Label(r, text="alder?").grid(row=2)
alderlab.pack()
navnlab.pack()
navn = Entry(r)
alder = Entry(r)

navn.grid(row=0, column=10)
alder.grid(row=1, column=10)

r.mainloop()

• Change all single letter variables to meaningful names. Though not strictly required,
  it's better code, and will make debugging much easier as the program grows. (r and f)
  
• line 6 - you have already instantiated Tk, only one allowed per tkinter script remove this one
  
• line 7 - frame is never used
  
• line 12 - function never used, and wouldn't work at any rate
  
• line 13 - what is pack_forget??
  
• line 21 - what is compound? there is no such attribute for Button widget
  
• line 22 - cannot use grid, place and pack in same widget choose one and use throughout
  
• line 23 - same as 22
  
• ... more apply fixes above to rest of code.