The class of the blue wire (Character Movement Component) and the input pin class (Floating Pawn Movement) are unrelated and you can’t simply plug this in.
What you probably want is the following (see image):
Drag your wire into empty space and search for “set max speed” and pick the appropriate function based on the movement mode your character is in.
*If this is not you want and you actually wanted to use Floating Pawn Movement, your wire is wrong; but that’s hard to judge based just on this snippet, so screenshot some wider context in that case please. However, if I had to guess, I’d say the thing I said previously is more likely the thing you want.
Do Not change a characters movement speed from another class.
Movement speeds in the character class have to be applied from within the class on the local client, then the servers copy has to make the same changes. Otherwise the server will correct and overwrite the change on the client.
Not sure where in the provided context you see a mention of setting “movement speed” or networking, pretty sure they’re just trying to limit their character’s max speed. “Setting movement speed” as a concept doesn’t even exist, speed is derived from velocity. Also, the speed limits aren’t even replicated.
Let’s just stick to the context they provided and see if they simply used the wrong node. 
Your pic is casting to the character movement component. You do not need to do that in the character class. This infers you are not in the character class and attempting to modify CMC outside of the owning class. Your pic also highlights CMC movement speeds.
We are in the multiplayer section, thus networking. If you are applying movement speed changes in multiplayer on CMC you have to do it the way I noted.
I casted just to get the proper nodes offered for the class the OP had, that’s why my snippet half-hides the cast node. They already have an object of that type, the cast is irrelevant.