You need to book an appointment of nearest centres from 5 preferred option and assign preferences

You can then go for mock exam at confirmed centre by NTA for mocks.

https://www.timesnownews.com/education/article/nta-mock-test-centres-for-ugc-net-dec-2018-and-jee-main-2019-exam-on-nta-ac-in-steps-to-book-a-mock-test-slot/281000