Well she is probably thinking it is OK because it is outside. Also are you sure you want a dog using the lawn? A lot of us actually prefer dogs to go on hard surfaces and not lawns. It is a lot easier to scoop poop on a hard surface and hose off urine, whereas on a lawn it causes lawn burns and can be very hard to lift.
Wherever you decide on, you will need to take her out every single time, on a lead, to the place you designate her official poop area if you want to train her to go in a specific place. Then you need to praise her and reward *every time* until this becomes a firm habit. If she is still going inside it is most likely because you gave her too much freedom too soon, assuming by some early successes that she was housetrained (as so many of us mistakenly do!).
It helps to keep a small bit of stool at the place you want her to go as this helps trigger her to go there again and seek tis place out.
Remember a dog will only do what it has clearly been told to do and motivated in a positive way to do. Housebreaking takes a huge amount of work and it must be consistent and focused.
Sometimes it helps to think of what you'd do if you were potty training a two year old that spoke not a word of the language you speak. That is what the situation is like for a dog! Over and over, you have to be there to convey the message of where to go and your delight at the message being correctly understood. Over and over and over!
Eventually it makes sense to the dog and the dog matures enough to remember what it is supposed to do -- something most puppies just cannot do right away (like a toddler). Most of us find it takes until the dog is about 6 months old to be about 85% housetrained and then it is fine tuning with accidents now and then til around age 1.
I strongly recommend the Shirlee Kalstone book as it will give clear guidance and schedules.