It’s a message from the command line program pip, which Rapid Photo Downloader calls as it determines if a newer version is available for download. You can ignore it.
From a virtual machine, with LinuxMint 18.1:
damon@Mint18 ~ $ echo $PATH /home/damon/bin:/home/damon/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
Have your made your copy of LinuxMint fully up-to-date?