get.yield.equation <-function(t, power, flow, PV){
sum(flow/((1+t)^power)) - PV # t is a parameter to be calculated
}

power<-c(0.01549014, 1.01549014)
flow<-c(0.5, 100.5)
PV<-67.07738

f.ytm<-function(t) {0.5/ (1+t)^(0.01549014)  + 100.5/ (1+t)^(1.01549014) - 67.07738 }
uniroot(f.ytm, interval=c(-1,1), tol= 0.000000000000000001)$root
[1] 0.5

t<-uniroot(get.yield.equation(t, c(0.01549014, 1.01549014), c(0.5, 100.5), 67.07738), interval=c(-1,1), tol= 0.000000000000000001)
Error in uniroot(get.yield.equation(t, c(0.01549014, 1.01549014), c(0.5,  :
could not find function "f"

uniroot(get.yield.equation(c(0.01549014, 1.01549014), c(0.5, 100.5), 67.07738), interval=c(-1,1), tol= 0.000000000000000001)
Error in get.yield.equation(c(0.01549014, 1.01549014), c(0.5, 100.5),  :
argument "PV" is missing, with no default