Using OpenGL 4.6, I have the following (abbreviated) code, in which I create a buffer and then attempt to map it in order to copy data over using memcpy():
glCreateBuffers(buffers.size(), buffers.data()); // buffers is a std::array of GLuints
// ...
glNamedBufferStorage(buffers[3], n * sizeof(glm::vec4), nullptr, 0); // I also tried GL_DYNAMIC_STORAGE_BIT
// ...
void* bfrptr = glMapNamedBuffer(buffers[3], GL_WRITE_ONLY);
This latter call returns GL_INVALID_OPERATION. I am sure that this is the call that generates the error, as I catch OpenGL errors right before it as well. The manpage suggests that this error is only generated if the given buffer handle is not the name of an existing buffer object, but I'm sure I created it. Is there anything else I'm missing or that I'm doing wrong?
When you create immutable buffer storage, you must tell OpenGL how you intend to access that storage from the CPU. These are not "usage hints"; these are requirements, a contract between yourself and OpenGL which GL will hold you to.
You passed 0 for the access mask. That means that you told OpenGL (among other things) that you were not going to access it by mapping it. Which you then tried to do.
So it didn't let you.
If you want to map an immutable buffer, you must tell OpenGL at storage allocation time that you're going to do that. Specifically, if you want to map it for writing, you must use the GL_MAP_WRITE_BIT flag in the gl(Named)BufferStorage call.