Hello, 

Try doing it agains. use a different laptop or system may be their is an issue with it.

sometimes the server is low which causes such issues.

Wait for 1 or 2 days or try after few hours and if still it causes an issu then I thinking calling the auntority would be the best option as the last date to fill the form is Feburary 21, 2019 (tentatively)

https://www.careers360.com/exams/bhu-uet