This comes from here then: You are using a special template that is lacking the necessary definition for \tightlist command. This is used by Pandoc (depending on the type of list you are using - they can be tight or not I think)
See the default Pandoc's template you need to build on
You need to add those line in your custom template. (and maybe more depending on the feature you are / want to use). By using a custom template, you are in advanced mode and need to adapt it yourself based on Pandoc feature, with the pandoc version you are using.
It is easier to tweak the default template by inserting preamble, before body or after body, and activating / deactivation feature using variables.
Anyway, this should solve your issue