Starting the development

I started using nymea, and I’m really eager to start making a plugin. The problem is that I stuck on setting up the development environment.
Is there an image file, that contains everything to start programming?
I was trying to get the repo but with error:
Ign:1 InRelease
Err:2 Release
404 Not Found [IP: 80]
Hit:3 bionic InRelease
Hit:4 bionic-updates InRelease
Hit:5 bionic-backports InRelease
Get:6 bionic-security InRelease [88,7 kB]
Reading package lists… Done
E: The repository ‘ Release’ does not have a Release file.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Also, if you could help me, on how to test the written code.
I know that this sounds very very noob, but I want to learn.
Thank you

Your file /etc/apt/sources.list.d/nymea.list
should look something like this:

   >  ## nymea repo
   >  deb bionic main
   >  deb-src bionic main

if you had to change the file do:

> sudo apt update

To install and test a plug-in, I do:

$ mkdir build
$ cd build 
$ qmake ../
$ make
$ sudo make install
$ sudo systemctl stop nymead
$ sudo nymead -n -p

See also:

Thank you, I was using wrong. my bad

No worries, don’t hesitate if you have further questions