See the FAQ: How to do a minimal reproducible example reprex for beginners. Could you reframe the question along the following lines?
# constants
L <- 1.210^(-8)
u0 <- 1.610^(-19) * 23210^(-3)
m_star <- 9.10910^(-31)
hbar <- (6.6310^(-34)) / (2 * pi)
me <- 0.0679 * 10910^(-31)
mh <- 0.59 * 10910^(-31)
theta0 <- sqrt(((m_star) * (u0) * (L^(2))) / (2 * (hbar^(2))))
generate_theta <- function(k) (k*L)/2
plot(generate_theta(1:100),theta0*cos(generate_theta(1:100)))
