Hi
The issue may be that mpd_oled isn’t being run with root privileges. I installed the plugin and see the error for an SPI OLED. If I run the mpd_oled binary directly without sudo I get the same error. If I run it with sudo it runs correctly.
Test if this is the issue by running, from the Volumio home directory (-o 7 for SH1106 SPI OLED)
sudo mpd_oled/mpd_oled -o 7
On the other hand, I2C displays will probably run without root privileges.
Adrian