Signature check failed - followed the tutorials

I did exactly as they say in the guides, installed VSCode, Monkey C extension, generated developer key, verified that the extension settings are right, built my app, a watchface, works fine in the simulator, but when side-loading using MTP -> /GARMIN/Apps, it get an error and it the watch face is not listed. When I re-mount the watch, the .prg file is gone, and there's new entry in the log with Signature check failed.

This is the command VSCode is running to build the app:

java -Xms1g -Dfile.encoding=UTF-8 -Dapple.awt.UIElement=true -jar /home/skrat/.Garmin/ConnectIQ/Sdks/connectiq-sdk-lin-7.1.0_RC2-2024-02-14-4a71df847/bin/monkeybrains.jar -o /home/skrat/SexyFace.prg -f /home/skrat/Workspace/Garmin/SexyFace/monkey.jungle -y /home/skrat/.Garmin/developer_key -d fr255sm -w -r

WARNING: fr255sm: /home/skrat/Workspace/Garmin/SexyFace/resources/layouts/layout.xml:8: The resource expression '-5%' in attribute 'y' for resource id 'TimeLabelHrs' could not be validated. Consider using formats: center, start, top, bottom, left, right, a valid pixel value, a valid relative position.
BUILD SUCCESSFUL

Note that event the simulator accepts and does the right thing with negative y values.