That variable is defined in the 2nd code cell:
That error message indicates either the above cell isn’t executed or isn’t successfully executed. Please try to re-run all the code cells starting from the very first one and make sure each and every one is successful.
PS: It’s always safer to run starting from the first code cells every time we restart a jupyter notebook.
That worked. Thank you very much.
You are welcome @Begench_Gurbanow!