I have planted this red edible amaranth and it has started blooming without even reaching the adequate height. I have not had access to fertiliser due to corona lockdown. Is that the reason for this or something else?
It is probably due to inadequate soil, as you have suspected. The best amaranth grows right next to a compost heap and will easily go to 2 metres in height. Increase the size of the pot and use about 10 times as much soil (sandy type soil from the yard would be good) and pinch back the flowering head to force it back into a vegetative state. If you have any farms local to you try to find some manure, make some compost tea, use it in watering and let the pigweed rip.