The bug

Saving a structure which has not been saved before logs an error with the message "Couldn't load structure". This should not be the case since you are saving and not loading a structure.

How to reproduce

1. /setblock ~ ~ ~ structure_block[mode=save]{mode:"SAVE",name:"minecraft:new_structure"}
   

2. Open the structure block GUI and click "Save"
3. Look at the log
   → An error with the message "Couldn't load structure" has been logged