Did you make sure to install the pull-up resistors on the 16n?
From the build guide:
By default, I2C is not set up for ‘master’ mode (ie: it will work with monome Teletype and similar, but not ER-301/Ansible. To make it work for ER-301, you need to enable ‘master’ mode in
config.h, and ensure your pull-up resistors are in place).