What's wrong

wps reports a useless else clause after a for, but I don't think the clause is useless: I don't see any way to remove it without writing more code:

for search_path in self.environment.loader.searchpath:
    template_relative_path = Path(search_path) / relative_path
    if template_relative_path.exists():
        break
else:
    raise UserMessageError(

Expression complexity should be integer, in code and in error text. Now it is float.

Classes Num, Str, Bytes, NameConstant, and Ellipsis are considered deprecated in favor of ast.Constant since python 3.8.

Consider something like:

isinstance(node.left.value, (bytes, str))

See: https://bugs.python.org/issue32892