The definition for std::format is as the following

template<class... Args>
string format(format-string<Args...> fmt, const Args&... args);

template<class... Args>
wstring format(wformat-string<Args...> fmt, const Args&... args);
// .....

In this definition, the format-string or wformat-string is an alias template, defined as the following

template<class... Args>
using format-string =   basic-format-string<char, type_identity_t<Args>...>;                      // exposition only
      
template<class... Args>
using wformat-string =  basic-format-string<wchar_t, type_identity_t<Args>...>;                            // exposition only
     

where basic-format-string is a class template, It arguably says the first parameters of std::formart in this two cases are class template specialization. We also gives a simple use of std::format in the subsequent clauses, which is

string s0 = format("{} to {}",   "a", "b"); // OK, automatic indexing

The first argument has an array type, in template function call deduction, it is decayed to a pointer to participate the deduction, which means basic-format-string<char, type_identity_t<Args>...> will compare with pointer to const char, as per [temp.deduct.type#2], the deduction on this pair cannot be done. The deduction will fail.