Problem not following A SelectorLink?

I am trying to work out why my webScraper does not want to follow my "Chapters" in my below code as it then does not carry out the last scrape on the videourl

If any one could please help as its all working apart from this last url follow

Sitemap:
{"_id":"Downloadmylectures","startUrl":["https://www.myschool.ac.uk"],"selectors":[{"id":"CourseLink","type":"SelectorLink","selector":"tr:nth-of-type(n+2) a:nth-of-type(2)","parentSelectors":["root"],"multiple":false,"delay":0},{"id":"CourseName","type":"SelectorText","selector":"span.syllabus-title span","parentSelectors":["CourseLink"],"multiple":false,"regex":"","delay":0},{"id":"CourseMaterial_1","type":"SelectorLink","selector":"tr:nth-of-type(3):contains('') a.bookshelf-item","parentSelectors":["CourseLink"],"multiple":false,"delay":0},{"id":"CourseMaterial_2","type":"SelectorLink","selector":"tr:nth-of-type(5):contains('') a.bookshelf-item","parentSelectors":["CourseLink"],"multiple":false,"delay":0},{"id":"CourseMaterial_3","type":"SelectorLink","selector":"tr:nth-of-type(7):contains('') a.bookshelf-item","parentSelectors":["CourseLink"],"multiple":false,"delay":0},{"id":"Chapters","type":"SelectorLink","selector":"table.syllabus-table a","parentSelectors":["CourseLink"],"multiple":true,"delay":0},{"id":"VideoUrl","type":"SelectorHTML","selector":"#form1 > div:nth-child(8) > script","parentSelectors":["Chapters"],"multiple":false,"regex":"((?:https\:\/\/)|(?:http\:\/\/)|(?:www\.))?([a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,3}(?:\??)[a-zA-Z0-9\-\.\?\,\'\/\\\+&%\$#\=~]+)","delay":0}]}"}

Hello
"Json is not valid"
Impossible to import your sitemaps