Problem with PHP error

Hi,
I believe there is a problem with the .php for our website, every page displays correctly accept the home page which is white and does not load any data

below is the PHP errors I am getting
https://www.thechristmasdecorators.co.uk

[09-Jun-2022 08:35:17 UTC] PHP Fatal error: Uncaught Error: Call to undefined function ot_get_option() in /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-content/themes/decorators/header.php:69
Stack trace:
#0 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-includes/template.php(770): require_once()
#1 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-includes/template.php(716): load_template('/home/customer/...', true, Array)
#2 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-includes/general-template.php(48): locate_template(Array, true, true, Array)
#3 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-content/themes/decorators/page.php(14): get_header()
#4 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-includes/template-loader.php(106): include('/home/customer/...')
#5 /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-blog-header.php(19): require_once('/home/customer/...')
#6 /home/customer/www/thechristmasdecorators.co.uk/public_html/index.php(17): r in /home/customer/www/thechristmasdecorators.co.uk/public_html/wp-content/themes/decorators/header.php on line 69