Hi!
In ggplot2 you can plot different layers on top of each other. In your case you can at first plot the Plan bars and then the implementation bars on top. Here is an example:
suppressPackageStartupMessages({
library(dplyr)
library(ggplot2)})
mydf <- data.frame( Miesiac = c('styczeń', 'luty', 'marzec', 'kwiecień', 'maj', 'czerwiec', 'lipiec', 'sierpień', 'wrzesień', 'październik', 'listopad', 'grudzień')
, Plan = c(2000000, 2000000, 2000000, 2000000, 2000000, 2200000, 2500000, 2500000, 2500000, 2500000, 2500000, 3000000)
, Realizacja = c(1000000, 1200000, 1262076, 2000000, 2500000, 3500000,0,0,0,0,0,0))
# Add percentage column for labels
mydf<-mydf%>%mutate(percent=Realizacja/Plan*100)
# convert months? column into factor to preserv order
mydf$Miesiac<-factor(mydf$Miesiac,levels=mydf$Miesiac)
# plot
mydf%>%ggplot(aes(x=Miesiac))+
geom_bar(aes(y=Realizacja),stat = "identity",fill="lightblue")+
geom_bar(aes(y=Plan),stat = "identity",alpha=0,colour="black")+
geom_label(aes(y=Realizacja, label=paste0(round(percent,0),"%")))+
scale_y_continuous(labels = scales::comma)

Created on 2021-01-18 by the reprex package (v0.3.0)
Note that I filled in the Realizacja column with zeros, otherwise it would have been recycled.
Does this help you?