Can I say the following?
- If we don't fix this problem now, then our plan may jeopardize.
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Your sentence improperly treats jeopardize as an intransitive verb meaning come into jeopardy—that is, become exposed to risk. But jeopardize is a transitive verb meaning put [something] into jeopardy—that is, expose [something] to risk.
What you want to say is something like
Not fixing this problem now may jeopardize our plan.