Can you show the function where you actually call StartSprint and EndSprint?
What I suspect the problem is that the value only changes on the server and I don’t think MaxWalkSpeed is replicated, so only the server knows about the change. You need to make sure that both know it. So if you’re calling it on the client, call the server command and then immediately change it locally too.
Does that make sense? Seeing the code where you’re calling StartSprint will help out answering this question.