SimpleNote won't install

Aaaah, i may have found the hint why Gnome Software does not show simplenote.
From

Linux Mint supports Flatpak (Flathub) by default. But its Software Center only shows verified applications, so Simplenote is not available.

So it seems Gnome Software only shows verified apps too. Discover shows veriefied and unverified apps.

So everything good with your setup.

@Rowly It’s there in GNOME Software (well for me)…

But I normally use the command line (well user script) to add the flathub repo as well as install flatpaks all as my user, not system wide.

It’s really odd. When I installed Gnome Leap on my old laptop there was a pop up prompt at some point asking if I wanted to add the flatpak directory. I didn’t get it when I installed it on my newer laptop and reverted to a script I had used before. I did an update after installing the directory, perhaps this ballsed something up?
I will take another look in the morning. Atm I’m a bit fed up with having encrypted the laptop which means three log ins and quite a dely at the first. I may just strt all over again.

Malcolm, just an update, I reinstalled Leap 16 Gnome last night, updated immediately and added the Flatpack repo, the source I used added “sudo flatpak update - - appstream”. My Suse-fu then allowed me to smoothly do all the other bits (download, run and get working at startup the icedrive app image and get all the apps I require for this machine).
So my thanks to everyone for their help and advice.