Hi Paolo,
Can you explain in which circumstances the plugin functions are not loaded by default?

PS: I cannot find any information about that in the WP.org (https://developer.wordpress.org/reference/functions/get_plugin_data/).

PPS: OK, with all other plugins disabled, and with the default WP theme (Twenty Twenty-Two), I do get the following error:

Fatal error: Uncaught Error: Call to undefined function get_plugin_data()
...

:-( The WordPress developer documentation is full of traps... Not mentioning that this function may not be loaded by default (even in the admin) is a real pitfall.

-> as a fix, I would rather defining the RPBCHESSBOARD_VERSION directly instead of relying on such poorly documented mechanism.

@paolofantozzi I've issued a new version that fixes the above-mentioned error, using a different fix than what you proposed. Still, thanks for reporting.

Thank you for your efficiency!