constexpr const vs constexpr variables?

The issue is that in a variable declaration, constexpr always applies the const-ness to the object declared; const on the other hand can apply to a different type, depending on the placement.

Thus

constexpr const int i = 3;
constexpr int i = 3;

are equivalent;

constexpr char* p = nullptr;
constexpr char* const p = nullptr;

are equivalent; both make p a const pointer to char.

constexpr const char* p = nullptr;
constexpr const char* const p = nullptr;

are equivalent. constexpr makes p a const pointer. The const in const char * makes p point to const char.

The error message you're seeing has nothing to do with the constexpr keyword per se.

A string literal like "foo", as in:

somefunction("foo");

The type of this string literal is const char *. The following statement:

char *const str = "foo";

This tries to assign a const char * value to a char * value. The resulting char * value is non-mutable, constant, but by that time the error already occured: an attempt to convert a const char * to a char *.

The constexpr keyword in your example is just a distraction, and has no bearing on the error.