Hi bh21,
First, my pleasure to assist you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
Based on my test result, I’m able to see the “Validation settings” option in a list setting. See image below for your reference:
Since you are not able to see “Validation settings” option in your list setting, if you don’t mind (my request please don’t mind) could you please confirm what permission do you have in SharePoint online list. Based on my test result if the user have "Edit permission" cannot see “Validation settings” option in a list setting. See my image below for your reference:
Note: Because the Full Control permissions are needed for “Validation settings” option in your list setting,
Regarding to your need, "I want to add validation so that Column B cannot be filled unless column A is ticked." If you don’t mind, could you please try to access the Validation Settings page by manually entering the URL. For example:
While on the list settings page, your URL will contain:
.../_layouts/listedit.aspx?List=%...
If you change this to:
.../_layouts/VldSetng.aspx?List=%...
Then you will be on the Validation Settings page for the same list.
I appreciate your understanding and stay safe!!
Best regards
Waqas Muhammad