> m <- manova(U~factor(rep(1:3, c(3, 2, 3))))
> summary(m,test="Wilks")
Df Wilks approx F num Df den Df Pr(>F)
factor(rep(1:3, c(3, 2, 3))) 2 0.0385 8.1989 4 8 0.006234 **
Residuals 5
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
You forgot to declare 1:3 to be a factor.