My DataFrame keeps getting canceled

While the side loading supposed to work, you can also try to upload it as a beta app.

5923570 said:

I follow the procedure of building for device via Monkey C on VSCode, and copy the .prg file in the folder Garmin/APPS. 
When removing the USB connection, the .prg file is filtered and put in a file OUT.BIN. 

Modern Garmin watches "hide" PRGs (actually - move them to a user-inaccessible part of storage), whether sideloaded or installed from the store. This started as an apparent security measure for CIQ music providers (when they were introduced) and was eventually extended to all CIQ app types.

So the fact that your PRG disappears after copying it to /GARMIN/APPS is not indicative of a problem.

However, it is possible that the sideload failed for some reason. Take a look at /GARMIN/APPS/LOGS/CIQ_LOG.YML. Do you see an error with the phrase "signature check failed" (or something similar)? That would indicate a sideload failure (although I'm not sure why it would be failing in your exact case).

Another (unlikely) possibility is that you are already at the limit of installed CIQ apps. In this case, sideloading will silently fail. This has actually happened to me because I have way too many CIQ apps installed - the obvious workaround is to uninstall another app to make room.

I just came to open a fairly different discussion, but this response is kinda the answer I was after As a general comment, I observe that the outcome of side-loading is quite random; sometimes the app gets picked up, other times it disappears completely. There's definitely something going on there where a copy remains installed somewhere and and needs clearing via Garmin Express.

I do see these "Signature check failed on file: myfile" lines in CIQ_LOG.YML, and presumably they coincide with times the app wasn't successfully loaded. Any idea why/when that occurs? Is it transient?