Hi Elham,
Not sure what you mean by "panel data". Maybe you mean Dataset
? If so, this should get you started.
dataset = Import["~/Downloads/Book1.xlsx", {"Dataset", 1}, HeaderLines -> 1]

Group by year and compute mean of perco2
.
meanCO2ByYear = dataset[GroupBy["year"], Mean, "perco2"]

Plot it
meanCO2ByYear // Normal // ListLinePlot
