pushing 8 chars string on stack in x64 assembly

Question

I am working on a very basic 64 bits intel program.

I want to push some chars on stack.

It works fine if i do like this:

mov rcx, 'messsage.'
push rcx

This is good because my string message is 8 bytes length (64 bits). It perfectly fits in rcx register.

Now, it does not work if i do this:

push 'message.'

or

push qword 'message'.

I do not understand why this is not possible. Because it is possible to push immediate values like this:

push 0xFFAA0000FFAA0000

I just want to understand.

Thanks