I am using 60kg undermodified 6 row barley for 300 litres, I performed 60 minutes rest at 64°C and 30 minutes rest at 72°C, but I got 12°Plato original gravity (1.048) after boiling.
Please help me to get a higher gravity.
Undermodified 6-row barley by itself doesn't have enough enzymes to give you complete conversion of starches to sugars, so you end up with low gravity. Add some highly modified malt (i.e. one with high diastatic power) to help out the 6-row. Here is an article that may help: https://byo.com/stories/issue/item/3604-brewing-by-ratio
With all additional information posted in the comments, I will try to answer the question.
First, 12°Plato is equal to a SG of 1.048. However, the gravity was taken at 30°C, so using this calculator we end up with a corrected SG of 1.033.
When mashing using 6 row malt, a 30 minutes protein rest shoud be done between 48°C and 54°C, see How to brew for more information. 300L is a lot, make sure to mix it all well to even the temperature.
Then you can raise temperature to about 65°C for 60 minutes.
Temperature close to 76°C is only for a mash out (stopping the enzyme activity).
The second thing to watch is the water pH if your source of water is hard, but the mash temperature is most probably your problem.
As mentionned by GigaFemto, I just realised it is barley and not malt, so yes please add some 2-row/6-row malt to increase the diastatic power in the mash.