|There are a few issues here.|
1) What version of OpenVMS are you using. The limits change depending on version.
2) There is not enough info here to be 100% certain about the problem.
For a start:
DCL symbol substition happens prior to the actual write. From the message you posted, it would appear that the result of that substitution exceeds the maximum length of a symbol. This can be addressed a number of ways again depending upon the OpenVMS version.
Please update this with the version information and a small reproducer and further details and potential solutions can be provided.
P.S. From the HELP/MESSAGE command:
An element (that is, any string placed between two delimiters) in the command line exceeds the maximum length of 255 bytes.