MACBOOK AIR 2017 SHUTS DOWN WHEN AT 34%

Go to About this Mac >> System Report and on leaf look at Battery. In Right panel - how many Battery Cycles and Battery Health - Normal OR Needs Service ? If second one - suggest making appointment to have it services Only by Apple to Apple Authorized Service Centre

It, shutting down when when battery is low may be a symptom of the battery is loosing Capacity and may soon need replacement. Keep an eye one this before it becomes critical.

Good Luck