The very simple solution for this is to initialize some variable at the before the loop kicks in:
choice=''
#This means that choice is False now
while not choice:
choice=input("Enjoying the course? (y/n)")
if choice in ("yn")
#any set of instructions
else:
print("Sorry, I didn't catch that. Enter again: ")
choice=""
What this while conditional statement means is that as long as choice variable is false--doesn't have any value means choice=''-- ,then proceeds #with the loop
If the choice have any value then proceed with enters the loop body and check
the value for specific input, if the input doesn't fulfill the required value
then reset the choice variable to False value again to continue prompts user
until a correct input is supplied