Here is an example of using mtext() and messing with the mar parameter of corrplot() to make enough margin around the plot to place the labels. mar is one of the many parameters that can be set with par() function. If you look in the help for that function, there is a small explanation. I cannot explain why I had to set the line argument of mtext to 4 in one case. I just kept incrementing it until I got what I wanted.
library(corrplot)
#> Warning: package 'corrplot' was built under R version 3.5.3
#> corrplot 0.84 loaded
set.seed(5389)
x <- runif(2500, min = 0, max = 1)
CorrMat <- matrix(x, nrow = 50)
corrplot(CorrMat, is.corr = FALSE, tl.cex = 0.5, method = "color", tl.col="black",
mar = c(1,1,1,1))
mtext(text = LETTERS[5:1], side = 2, line = 0, at = seq(5, 45, 10), las = 1 )
mtext(text = LETTERS[1:5], side = 1, line = 4, at = seq(5, 45, 10), las = 1 )

Created on 2019-10-08 by the reprex package (v0.3.0.9000)