Stimulus 600

I file my 2020 taxes and have received my refund but I didn’t get the 600 stimulus that I was missing 

so how to I get it now I was told I was supposed to put it on line 30 of the tax form but it didn’t tell me that 

I was ask question and I answered that I received the 1200 but the the 600 

so how do I get the 600 now