Problem with Customizer and adding new pages

First off I am not a developer.

Have been creating a simple brochure style website just with some text editing and images, no coding.

Almost finished the build and I suddenly cannot add any pages. I go to add new page and get an error reading:

Warning creating default object from empty value in home …..
Warning Cannot modify header information – headers already sent by ….

I also can make changes which show in the preview of customizer and then will not save. I get changeset-post-save-failure.

I haven’t done anything other than basic things so cannot understand how it would have gone so wrong. This is the second time I have built the site for this to happen several hours in. Spoke to siteground support who say it’s the theme. Before I give up completely does anyone have any ideas?

Thanks in advance.