You shouldn’t be using the tilde here (“~”). Tilde suggests that the statement relates to a likelihood or a prior probability. However, you simply intend to define gamma, for which you should use “=”:
`gamma[i] = alpha + beta *COV[i];"
Also, this means that gamma should be either a local parameter/variable in the model block or, or a transformed parameter in the transformed parameters block. It should no be defined in the parameter block. Only alpha and beta are parameters that need to be estimated. Gamma is derived from that