Hello, thanks for read me

I don't know why my code don't work, I'm trying to calculate the magnitude of a complex vector but I get a error in the next image you can view it, 

Can someone help me? thank you

I followed the steps given in the example of Maple help "Example 5: Adding a MathMLViewer Region" but I get the next error window.

Do you know what is my mistake?

thank you.