Add equation to regression line on plot

Ok, so I want to add regression line equations to my plot. I found this answer which won't work for me for some reason. This is what my data looks like:

>Plot_Data Treatment value Substrate 1 Control 0.16666667 10.00000 2 Control 0.03333333 2.00000 3 Control 0.02380952 1.00000 4 Control 0.01388889 0.50000 5 Control 0.01250000 0.25000 6 Control 0.01219512 0.12500 7 Control 0.01176471 0.03125 8 +Inh P 0.50000000 10.00000 9 +Inh P 0.14285714 2.00000 10 +Inh P 0.10000000 1.00000 11 +Inh P 0.08333333 0.50000 12 +Inh P 0.07142857 0.25000 13 +Inh P 0.06666667 0.12500 14 +Inh P 0.06250000 0.03125 15 +Inh Q 0.43103448 10.00000 16 +Inh Q 0.08403361 2.00000 17 +Inh Q 0.05494505 1.00000 18 +Inh Q 0.02610966 0.50000 19 +Inh Q 0.02000000 0.25000 20 +Inh Q 0.01470588 0.12500 21 +Inh Q 0.01265823 0.03125 

And I use a slightly modified version (I added y and x as input) of the function suggested in the awnsere:

lm_eqn <- function(y,x,df){ m <- lm(y ~ x, df); eq <- substitute(italic(y) == a + b %.% italic(x)*","~~italic(r)^2~"="~r2, list(a = format(coef(m)[1], digits = 2), b = format(coef(m)[2], digits = 2), r2 = format(summary(m)$r.squared, digits = 3))) as.character(as.expression(eq)); } 

I then plot my graph with:

Plot <- ggplot(Plot_Data,aes(x=Substrate,y=value,group=Treatment,color=Treatment))+ geom_point(shape=1)+ geom_smooth(method = lm,fullrange =T,se=F,size=0.75)+ xlab(expression("[S]"^-1))+ ylab(expression("V"[0]^-1))+ xlim(c(-1.5,10))+ ggtitle("Adenylate Kinase rate graph")+ theme(axis.title = element_text(size=12), plot.title = element_text(hjust = 0.5))+ geom_text(x=5,y=0.5, label = lm_eqn(Data.Inverse$Substrate,Data.Inverse$X.Inh.P,Data.Inverse),color = "red") 

But I get the following output as a string of text without modifications:

Any ideas why? It seems the expression function isn't working properly but I don't get why.

Edit:

Data.Inverse is the dataframe Plot_Data was melted (also with Substrate added using rep and mutate) from, and it looks like :

Substrate Control X.Inh.P X.Inh.Q 1 10.00000 0.16666667 0.50000000 0.43103448 2 2.00000 0.03333333 0.14285714 0.08403361 3 1.00000 0.02380952 0.10000000 0.05494505 4 0.50000 0.01388889 0.08333333 0.02610966 5 0.25000 0.01250000 0.07142857 0.02000000 6 0.12500 0.01219512 0.06666667 0.01470588 7 0.03125 0.01176471 0.06250000 0.01265823