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I formatted your code so it's readable. And by indenting it, you can see where the problem is. It's in line 6/7 in the "FROM" part. Oracle can't really figure out what you're trying to do with that statement and when the parser reaches the end of the query at "(+)", he tells you that there is something missing/awkward. That's why he tells you it's the "(+)". But the problem is earlier. It's the way your build your "FROM" part. It's a bit confusing, so here is just a pointers:

when using "union", you don't give an alias to each of the two queries. The result set of both queries is "unioned", so you can't reference either of them in the "WHERE" part.

You should rewrite your query and instead of joining Client with a Union, you should Union two separate queries: one joining Client with casual_jobs, one joining Client with contrac_job. Following example is done without testing it in an SQL tool:

I would look into "polygon offsetting", the basic idea would be that for every vertex of the polygon you change that into a circle with a given radius, for every segment of the polygon you change that into a rectangle parallel to the original segment, and take the union of all resulting forms.