You can also use backticks around the column names that would otherwise be recognised as numbers.

from pyspark.sql import functions as F
 
unpivotExpr = "stack(3, '2018', `2018`, '2019', `2019`, '2020', `2020`) as (Year, CPI)"
unPivotDF = df.select("Country", F.expr(unpivotExpr))
unPivotDF.show()