Theme throws fatal PHP error and breaks site

Hello. I’m using Avante Lite v1.1.3 with WP v6.0.1. I tried to register for the forum at themely.com, but I’m getting an error with my IP address and nobody has replied to my request for access in a week. I tried the “contact us” link, but it just spins and does nothing when I press Send.

My client’s website was working fine when I set it up a year ago. Last week we discovered it only displays the first two sections of the home page, and then nothing. I found the following fatal PHP error in the log:

PHP Fatal error:  Uncaught Error: Undefined constant "adaptiveHeight" in /var/www/vhosts/example.com/httpdocs/wp-content/themes/avante-lite/sections/testimonials.php:8
Stack trace:
#0 /var/www/vhosts/example.com/httpdocs/wp-includes/template.php(772): require()
#1 /var/www/vhosts/example.com/httpdocs/wp-includes/template.php(716): load_template()
#2 /var/www/vhosts/example.com/httpdocs/wp-includes/general-template.php(204): locate_template()
#3 /var/www/vhosts/example.com/httpdocs/wp-content/themes/avante-lite/template-onepage.php(17): get_template_part()
#4 /var/www/vhosts/example.com/httpdocs/wp-includes/template-loader.php(106): include('...')
#5 /var/www/vhosts/example.com/httpdocs/wp-blog-header.php(19): require_once('...')
#6 /var/www/vhosts/example.com/httpdocs/index.php(17): require('...')
#7 {main}
  thrown in /var/www/vhosts/example.com/httpdocs/wp-content/themes/avante-lite/sections/testimonials.php on line 8

Is there something you can suggest I do while I wait for you to fix this problem with your theme, so I can get the site back up and running?

Please let me know if you need more information from me.

Thanks,
Lisa