When I do this
data_mibor= data.frame(
date = 1:5,
mibor_1m = (1:5)/2
)
plot(data_mibor$date, data_mibor$mibor_1m, type = "l",col = "blue", xlab = "Year", ylab = "Rate", lwd= 3,
main = "MIBOR_1m")

Created on 2020-06-10 by the reprex package (v0.3.0)
the plot is shown without an error, so you syntax is okay.
Your message suggests that data_mibor$date and data_mibor$mibor_1m have different lengths.
Check this by comparing length(data_mibor$date) and length(data_mibor$mibor_1m)
and find out what went wrong.