Walmart chlorine vs Chlorinator

[PoolDoc]

Is it ok to use the liquid chlorine with the automatic chlorinator?

You can use bleach (aka 'liquid chlorine') with an automatic chlorinator --but don't pour it through the skimmer.

why is chlorine not holding? is it because the other chemicals are off?

Probably.

How much liquid chlorine do I use and do I spread it over the deep end or in skimmer in this case?

A 20x40 vinyl is usually around 30,000 gallons, with a PF of 4. Since 6% bleach is equivalent to 1/2 lb of 100% chlorine gas, 1 gallon will add about 2 ppm of chlorine to your pool (4 x 0.5). So, while you are having problems, adding 3 gallons (6 ppm) of bleach to your pool each evening will probably keep you out of trouble. Just pour it in to the deep end, when the pump is running.

Meanwhile, read the test kit info page (link in my sig) and get EITHER an HTH 6-way drops kit (no strips!) if your Walmart happens to have it, or a cheap OTO (yellow drops) kit. Then tell us what your pH and chlorine level are. If you get the 6-way, tell us what your CYA level is.