There was an error in an expression in encryption at Output Expressions : Neither variable(s) nor rule(s) found: expression

My guess: In your rule!ALM_UpdatedEncryption you might have defined parametres for any rule or function twice like  

a!urlForTask(taskIds: taskIds: 12345, returnTaskPathOnly: false)

check the design errors log file which might help you pin point issues like this.

We need to see the code of that expression.

no error in system logs

if(
  or(
    a!isNullOrEmpty(ri!message),
    a!isNullOrEmpty(ri!key)
  ),
  null,
  a!localVariables(
    local!message: char(code(ri!message)),
    local!max: 16,
    local!splitted: a!forEach(
      items: enumerate(
        ceiling(length(local!message) / local!max)
      ),
      expression: a!forEach(
        items: fv!item * local!max + 1 + enumerate(local!max),
        expression: index(local!message, fv!item, {})
      )
    ),
    a!forEach(
      items: local!splitted,
      expression: rule!ALM_Encryption(message: joinarray(fv!item), key: ri!key)
    )
  )
)

And "ALM_Encryption"?

its a aes-128 implementation and is lengthy. I have used the rule in interface and is working fine. Other rules are working fine in process model.

not the standard out put log refer to the design_errors.csv file. If this error is due to any syntax error this log should provide some insight.

My guess is that there is a duplicate parameter keyword somewhere. Like

expression: expression: append(.....)

This is not caught by the syntax checker.