Brms package in R. Error: The following variables can neither be found in 'data' nor in 'data2'

Modeling
2

I think you need those unknowns in the priors. So I have a model like this:

brm(bf(y ~ a*x^2 + b*x + c, a + b + c ~ 1, nl = TRUE),
            data = dat2, prior = prior2)

and prior2 looks like this

prior2 <- prior(normal(0,1), nlpar = "a") + 
  prior(normal(0, 2), nlpar = "b") + 
  prior(normal(0, 2), nlpar = "c")

Is that what you are talking about?