The Dependencies analysis shows there is a reduction pattern dependency in the loop. Enable vectorization using the directive !$OMP SIMD REDUCTION(operator:list).

Example

!$OMP SIMD REDUCTION(+:SUMX)
...

!$OMP SIMD REDUCTION(+:SUMX)
do k = 1, size2
    sumx = sumx + x(k) * b(k)
enddo