You don’t say whether your baby is breast or formula fed. Constipation around 6-7 months of age are often as a result of a change in formula (no 1 to no 2), or a change from breast to formula feeds. Both these scenarios lead to a temporary change of gut function as far as digestion and absorbtion is concerned.
I suggest that you add a little more water to your baby’s diet and use the solids that she can have.
Before using any laxatives, please consult your doctor. Most of these cases are short lived and will sort itself out in a few days or weeks.