We have started using OS2.0 features of sections on all pages it is working great.
But when we tried to add sections in the customer’s template like customers/account, customers/login, customers/register
.json is not working in that templates.
Did I miss something in the document or steps?
Any help would be great 
Similar https://community.shopify.com/topic/1233825
Great question! The JSON templates do support customer page types, so you expect to see themes using this template method on future versions of Shopify themes. https://community.shopify.com/post/1234188
And yes according to the docs
Although JSON templates differ from Liquid templates in their contents, they are still template files that support the following Shopify theme features:> > - All template types, except for gift_card and robots.txt.> - Alternate templates.> > https://shopify.dev/themes/architecture/templates/json-templates
What theme is this?
Are you making an actual JSON template and have deleted the original .liquid version?
Are you following the migration guide?
Or are you trying to add sections to a standard .liquid template?
What code is causing trouble?
etc etc
Let try again with a refreshed mind and given solutions.
I will get back to you on this.