I’ve just pushed a staging version of my site live where I switched from Divi to Generatepress theme. No other plugins changed, and I’m getting the error message here. Any idea why?
(generally) WP throws that error if it cannot communicate with the REST API, which can simply be a permalink issue.
First thing to try is regenerating permalinks, by going to Settings > Permalinks and Click Save Changes.
If the issue persists, go to the Tools > Site Health and check if there are any issues related to the REST API being reported.