There are character literals (with ') and string literals (with "). Character literals have one character. String literals are arrays of characters. You can’t write something like 'plus' because it has more than one character (well technically you can, but it’s a multi-character literal, but lets not go there).
Nonetheless, this wouldn’t make any sense because operationptr points at a single char object. A single char can’t contain the entire word plus.
If you want to be able to accept plus as input, then I suggest you start using strings. In fact, use std::string.
As a side note, you are using pointers and dynamic allocation far too often. You are also forgetting to delete the objects that you create with new – this is a memory leak. I imagine you have come from a language that uses new for all object creation. In C++, this is not necessary (and is not a good practice). Instead, you can declare objects like so:
float aptr;
There is no need to dereference this object. You can just use aptr directly as a float.