(HP48) Beginner questions about RPL

[Pälzer]

Hello
I have been using HP calculators (17b, 19b, 20s, 48sx, 48g) in the office for some time for general calculations. Especially the HP48 and I love the multi-line display and use the stack extensively.
I am mainly just a software user and know little to nothing about programming.
I would now like to write a few very small programs in RPL and have had a code created by Chatgpt regarding the Collatz function:

Code:

<<
->n 
{ } ->L
n 
WHILE 
DUP 1 > 
REPEAT
DUP ->L
DUP 2 MOD 0 = 
IF
2 / 
ELSE
3 * 1 +
END
END
->L
>>

Unfortunately, entering the code fails at the "1" (line 6).
Does anyone have time and can help me?
I am not interested in optimal code, but in learning to understand the code as a beginner.
Thank you very much