The capture template you linked to is the key here:
("j" "Journal" entry
Entered on %U
The date you are modifying is in the content of the entry: the %U gets replaced with the current timestamp which you are then modifying.
However the 'file+olp+datetree' ...
Your setting is incorrect, C-h v org-capture-templates:
When using several keys, keys using the same prefix key must be
together in the list and preceded by a 2-element entry explaining the
prefix key, for example
("b" "Templates for marking stuff to buy")
so you need to put something like ("p" "Project Tasks") right before your "pn" template, for ...
I received the exact same error on a fresh Ubuntu install using Emacs 26.3. In my case it was related to the ob-ipython packet, which requires ipython and jupyter to be installed, which I hadn't come around to doing yet. After a sudo apt install ipython jupyter the error was gone.