The variable was not found in the Variables collection

I am trying to run the job on development server ,which executes master package. the master package contains all the ssis execute tasks for the child packages, But the job fails with the following error message The variable "User::DS_Env_Dest" was not found in the Variables collection. The variable might not exist in the correct scope. Attempt to parse the expression "@[User::DS_Env_Dest]" + "initial catalog=xyz;-----;"" failed and returned error code 0XC00470A6. The expression can not be parsed. It might contain invalid elements or it might not be well formed. there may also be an out-of-memory error. the job ran fine few days ago and i haven't made any changes to the package since then. it's master , child package structure with environment variables assigned to the packages i've checked the master package for the User::DS_Env_Dest variable and does exist in the package i even evaluated and works fine. does this mean the error is in master package or any of the child packages?? please let me know how to resolve this error? am i looking past small error here??