Yaml is heavily indentation oriented.
There are yaml validators like: YAMLlint. It will also the intendation for you. But not the wrongly set header option.
You have two indents (spaces) to much before the dash, plus js: needs to be upped too. Also the header property is set wrong, check the docs - if you want the js in the header, it needs its own library definition:
mysubtheme:
css:
theme:
css/slippry.css: {}
js:
js/slippry.js: {}
dependencies:
- core/jquery
This will work ! (but is in the footer)
This is how the two library definitions will look like:
slippry-css:
css:
theme:
css/slippry.css: {}
and
slippry-js:
header: true
js:
js/slippry.js: {}
dependencies:
- core/jquery
You'll need then to add both libraries!
Maybe this will work too:
mysubtheme:
header: true
css:
theme:
css/slippry.css: {}
js:
js/slippry.js: {}
dependencies:
- core/jquery
To debug your libraries, use libraries_ui