How do I fix the errors I get after removing the comment field from a content type?
Given your error stacktrace:

Check function gavias_sanbro_preprocess_node() (Line: 287 in your gavias_sanbro.theme theme file).

This preprocessing function obviously tries to access the comment field without checking, whether it exists. As you uninstalled the field, it is causing your exception.

You may comment out, remove, or alter the code accordingly.