Perhaps you meant u[x, y]^(1.)
in the last equation, instead of u[x, y]^{1.}
?
However, I think only linear F
's are supported in the current version of NDSolve
for pde. Try
NDSolve[{Laplacian[u[x, y], {x, y}] == u[x, y]^2,
DirichletCondition[u[x, y] == x + y, True]}, u, {x, y} \[Element]
Disk[]]