What is causing my sever muscle pain in my lower both legs that make it impossible for me to walk.

Sciatica could be a possible cause. Whatever the reason though, you should get a doctor to check it out.

Leg cramps can be caused by calcium deficiency, for one. You could have tendonitis, Achilles heel. Many other possibilities.

It could be a circulation problem. You really should see a doctor.

Hamstrings come to mind. Torn ACLs?

varicose veins can cause this.

Whatever the precise cause, you will be greatly helped, nevertheless, through regular exercise, proper diet and doing relaxed backbone stretching.
I 'unlock' my back several times daily - have done for decades now - still works quite well.
Your mind governs your whole body and life, triggering your brain and the thoughts your brain physically synthesizes.
Your brain, however, directs your central nervous system and triggers, via the nerves, virtually all the actions you undertake.
The central nervous system - the nerves of the body, if you like - are continuous, effectively, with your brain. Those nerves run down through the insides of your spine. You need to keep them loosened up and active; they are alive - like you ... so you need to stretch 'every which way and loose'. I do this most by bending over to touch my toes or in that direction [I can't reach mine either!] and relaxing the entire spine by just hanging there in that position for some seconds, or stretching forward to open up the spine and stretch it. Then you need to tip yourself laterally sideways - to one side, then to the other ... this is best if you also upstretch your arms as it also opens up your shoulders and pectoral girdle.
There are various other tricks you will learn for yourself that will RELAXEDLY open up andstretch your spine in its various directions.
These moves will unlock many of your leg, back, shoulder and arm and neck problems.
Good luck with it.

BTW, the 'Herbalife' program is also very useful for getting rid of various aches and pains in the body. If you can afford, worth a try.

Calcium, Magnesium, Potassium, Electrolytes
all play an important role in Muscle work.
A mineral imbalance in these can cause this.
If you are not taking any of this, try mega dosing in balance with these.
We need a lot of calcium upwards toward 1000 mg per day.
i used to have severe leg cramps at night. Both calf muscles at the same time.
Since i have boosted my intake of these i have not had any cramps. (Serious leg cramps in both calf muscles, that would make me scream in pain until i could stretch out the cramps.)
For electrolytes eating more vegies and drinking G2 EVERYDAY for electrolytes.
Don't know if any of this relates to you, but maybe.
Oh also am taking a homeopathic remedy with Quinine for leg cramps...
Also, as has been mentioned,
poor circulation could contribute to your condition.
Definitely have your DR look into this.

I don;t want to scare you but my wife had this and it turned out to be peripheral vascular disease. Do you smoke?
See a doctor please.