Hi Marge
I downloaded a copy of the site so that I could investigate more easily.
Interestingly, the issue only occurs with the GeneratePress child theme active, not with the parent theme, although it is not obvious why.
Anyway, I didn't dwell too long on exactly what is required to reproduce the PHP notice, because it is clearly the result of a flaw in the GeneratePress theme code.
If you want to share details with them, the PHP notice is
PHP Notice: A non well formed numeric value encountered in /Users/nigelanderson/OnTheGo/Sites/c-marge/wp-content/themes/generatepress/inc/css-output.php on line 752
The offending line of code (line 752 in the above mentioned file) is:
$css->add_property( 'max-width', generate_get_option( 'container_width' ) + $content_padding, false, 'px' );
Note that it is trying to add an option value (for container width) to a variable $content_padding.
$content_padding is a number.
But generate_get_option( 'container_width' ) is not a number. On your site it is "1200px", which is a string.
You can modify the theme container_width option in the Layouts tab of Customizer, and seemingly it stores the option in a format that includes the px units, rather than simply as a number.
That's the root cause of the problem, even though I haven't looked at what in Blocks prompts that theme code to be run.