To get a single value out of a data frame, you can use square brackets. The variable Value will have the value 48.99.
Value <- eqs[eqs$period == as.Date("2019-07-01"), "energy"]
You may not want to use the mutate() function as that makes a whole new column and if you pass it a single value, the whole column will have that value.
library(dplyr)
eqs <- data.frame(
energy = c(48.99, 13.34, 2.67, 6.25, 4.93, 2.71, 4.73, 16.38),
period = c(seq(from = as.Date("2019-07-01"), to = as.Date("2019-07-08"), by = 'day')),
stringsAsFactors = FALSE)
Value <- eqs[eqs$period == as.Date("2019-07-01"), "energy"]
eqs2 <- eqs %>% mutate(ROI = Value)
eqs2
#> energy period ROI
#> 1 48.99 2019-07-01 48.99
#> 2 13.34 2019-07-02 48.99
#> 3 2.67 2019-07-03 48.99
#> 4 6.25 2019-07-04 48.99
#> 5 4.93 2019-07-05 48.99
#> 6 2.71 2019-07-06 48.99
#> 7 4.73 2019-07-07 48.99
#> 8 16.38 2019-07-08 48.99
Created on 2019-09-09 by the reprex package (v0.2.1)