Thanks for the report.
It appears that the GDB EXE required to run this is missing in the latest toolchain (7.3.0-atmel3.6.1-arduino7) for v1.8.3 of "Arduino AVR Boards"
It is available in (7.3.0-atmel3.6.1-arduino5) for v1.8.2 of "Arduino AVR Boards", and v1.8.6 of "Arduino megaAVR Boards", but understand using the latest AVR core is often desirable.
The options to resolve this are:
1) Roll back the AVR code to v1.8.2
Or
2) Install a secondary core with the same tools:-
Install v1.8.6 of "Arduino megaAVR Boards" from the Micro Explorer window this will add the toolset, and press ReScan when installation has completed.
Then if you right click on the project > Add > Local Board.txt
And add the below entry to the file, and change the "MyUsername" to match your setup:
tools\avr-gcc\7.3.0-atmel3.6.1-arduino5/bin
Now if you try attaching again this should work as expected.
We will update when we have an update which resolves this more easily.