If that key is taken by another mod or screenshot app, you can change it in the config using Generic Mod Config Menu or editing the mod's config.json file.
Screenshots are saved in your SDV folder under Screenshots. Files are named by save name and real time stamp.
Technical
Requires SDV 1.5.5+
Requires SMAPI, doesn't use Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.