This is very common error that may be accrued in your site mostly in WordPress site where the function session_is_registered is used. Mostly accrued in contact form or registration form.
Solution of This Problem
This function is removed as of version 5.4.0 PHP. So you have to downgrade the PHP version to run this function on your site,
How To DownGrade PHP?
for this purpose you have to contact your hosting provider and asking them about the downgrade, Mostly shared hosting will not allow you to downgrade or even they will not downgrade for you. if you are on your own Machine VPS, than you may contact management team of your VPS to downgrade the PHP version.