I have a site which was built several years ago. Recently I upgrade the WP to latest version and PHP at 7.4.11. When I visit the dashboard Toolset > Setting, I got REST API error message, although it looks not affecting the frontend, I am not sure whether it is a potential problem that I can foresee. I do update all the Toolset plugins to the latest version but Layouts and Views have been installed. There are a lot of Views on this site, do I need to replace it with the blocks? Can all the Views be kept if I install the blocks instead?
I expect that REST error message is because you are not using the latest version of Views. Layouts and Views have been temporarily removed from the automatic installer and need to be updated manually from the downloads page: toolset.com/account/downloads
You can switch Views to Blocks if you like (Blocks is included in the custom Toolset installer). They are basically the same plugin, but expose different interfaces by default for creating Views, templates etc. If you install Blocks instead of Views (they cannot both be active), then go to Toolset > Settings and from there you can specify that it should use the legacy Views interface, in which case the plugin should function the same as Views.
There is a minor update for Layouts available which you will need to manually install.
As for the fatal error you shared, first update the plugins to see if it is resolved. If not, it is coming from some custom code added to your child theme's functions.php file (line 35). Please share that section of code, we may be able to spot the problem.