You have not missed anything -- there is no way to attach a file to Stack Exchange posts. There is a limit; I could only put my hands on the Answer-body-limit, in the Complete Rate-Limiting Guide, under Answering, saying:
Maximum of 30000 characters in answer
(If anyone has a better reference, particularly for Questions, please update this post).
Your specific situation had a request for clarification on which systems you ran the command on and what commands you typed to encounter the error. Neither of those should push you into a character limit.
If, however, you do have a large amount of data (e.g. log files) to post, my suggestions would be:
- start by pasting all (or as much as you can) of it into the Question box
- format it appropriately (the curly braces, to format it as "code")
- If it's too large, remove lines that are repeated and insert a command saying "removed repeated lines"
- If it's still too large, scan for messages that do not contain error strings like "fail", "warning", or "error" and consider deleting them.
- If it's still too large, post the top 20-50 lines and the bottom 20-50 lines, and extend those arbitrary limits if you see changing errors.