I don't know if this is the right place to post this or not.
I was using the configurable blocks plugin to change the background color on the home page header block and I got a "Call to undefined function template_preprocess_block() " error whenever I tried to display the home page. I finally had to disable the "Configurable Block Style" module to get it to display again.
Hello @ChuckTilbury 👋
The best place to report issues for specific contributed modules is their issue queue on GitHub. If you go to https://github.com/backdrop-contrib you will find a search field that can be used to search for the project by name. Once you have found the correct project, click on the "Issues" link at the top of the page.
For the Configurable Block Style module specifically, the issue queue is https://github.com/backdrop-contrib/configurable_block_style/issues and I can see that the problem you are seeing has already been reported before: https://github.com/backdrop-contrib/configurable_block_style/issues/8