Different t-test results from R and LibreCalc
If alternative = 'greater' means that alternative hypothesis is "x has greater mean than y". In your code:

t.test(data[,1],data[,2],paired = TRUE,alternative = 'greater')

it translates to data[,1] has grater mean that data[,2]. Which is obviously not true. Just change the order of inputs and you'll get a correct p-value   

t.test(data[,2],data[,1],paired = TRUE,alternative = 'greater')

I suppose that in LibreCalc arguments are flipped w.r.t. R