Is this what you are trying to do?
library(data.table)
DT <- data.table(PERMNO = rep(c(10026, 10027), each = 3),
Date = rep(c(20180101, 20180102, 20180103), 2),
PRC = c(1,2,3,4,5,6))
DT
#> PERMNO Date PRC
#> 1: 10026 20180101 1
#> 2: 10026 20180102 2
#> 3: 10026 20180103 3
#> 4: 10027 20180101 4
#> 5: 10027 20180102 5
#> 6: 10027 20180103 6
DTwide <- dcast(DT, Date ~ PERMNO, value.var = "PRC")
DTwide
#> Date 10026 10027
#> 1: 20180101 1 4
#> 2: 20180102 2 5
#> 3: 20180103 3 6
Created on 2019-09-12 by the reprex package (v0.2.1)