Open your terminal and type the below command in terminal.
snap changes
you will see the following results
ID Status Spawn Ready Summary
3 Done today at 17:04 IST today at 17:10 IST Auto-refresh snaps "core18", "gnome-3-34-1804", "snap-store"
4 Done today at 19:59 IST today at 20:01 IST Install "stickynotes" snap from "latest/stable" channel
Note that id in this output . Here its 4(for sticky notes app). You need to abort that snap operation by running the below command.
sudo snap abort 4
After this try to install it again. If you still get that error restart your system and try again. And try to refresh your package also.
sudo snap refresh stickynotes
Thankyou , hope this helps!