Here's why and some options depending on the context:
* "Advisable" is already a complete adjective. It means something is recommended or suggested. Adding a prefix might change the meaning.
* You might need a different word entirely. Consider synonyms like "prudent," "wise," "desirable," or "recommended" depending on the specific nuance you want to express.
However, here are some prefixes you could use with caution:
* "Highly": This emphasizes the strength of the advice. (e.g., "highly advisable")
* "Strongly": Similar to "highly," this adds emphasis. (e.g., "strongly advisable")
* "Un-": This would create the opposite meaning, "unadvisable."
Ultimately, the best approach depends on the specific context and the meaning you want to convey.