how do i disable a checkbox?

[merry shale]

What are you using for this modal ? How are you creating it ?

[fluid wadi]

html and ts

creating by pressing a button

[tall pelican]

Since you didn't post any code, we'll have to assume that this modal content is inside a component, and that it uses an Angular reactive form control. And that whatever UI library you're using to open this modal gives you access to the component instance. So add a method to that component instance, or pass it data, so that it calls disable() on this form control.
If you need better help, then post a better question, by following the #how-to-get-help instructions: post code, be specific.

[fluid wadi]

What does the disable() function do?

Will that function be called in the ts file?

[tall pelican]

Your checkbox is apparently disabled if !bulkCompanyService.selectedCompaniesCount is true. If you want it to be disabled in other cases, then change that condition.