Look if checkboxis checked

Hello,
my plan is to include a checkbox in my app and save the checked options in a string. But anyway it don´t work. Anyway the id of my checkbox seems to be undefined… hope someone have an idea what i make false. thanks

start.html

start.ts
start_ts

and my error code:

possibility would be to set a global variable true or fals for each of the checkboxes. but it is really not the smartest idea - anyone any idea for an easier way?

document.getElementById is not the angular way.
Take a look at angular binding.

Or at the example in the ionic docs.

I worked with an online flexbox builder and used the code. All worked fine.