In Blender 2.79 and 2.80 on os Linux the command ALT+C (Convert to Curve|Mesh) does not work: pressing the keys does not cause any results. Note that in the preferences the shortcut is correctly set.
In Windows everything is OK.
Shortcuts on the OS take precedence over those in blender.
There are two options:
You can remap the shortcuts on linux to the super (windows) key.
You can customize the shortcuts on blender, just know that any books or "tutorials" you watch will not match with your personalized keyboards.