思いつかない vs.思いつけない: What's the difference?

結論からというと、

思いつかない！

There's no 「思いつけない」.

"思いつく" explains just one situation like "私はその時良いアイデアをおもいついた." I had a good idea at that time.

So this is just a statement of fact.

"Have" or "Have not."

Meaning: To think of; to hit upon; to come into one's mind; to be struck with an idea.

「思いつける」は、「思いつくことができる」

「思いつけない」は、「思いつくことができない」と記述するのが本来は正しい用法です。

~~You(or someone) can come up with a new idea~~

~~彼はあんな斬新なアイデアをおもいつけるなんてすごい！私には思いつけない。~~

~~Incredibly, he can think of such a novel idea! I can't think of it.~~

~~"Can" or "Cannot."~~

~~それを思いつけるかどうかが、天才と凡人の差だよ。Whether anyone can think of this is the gap between genius and mortals.~~

~~悔しいけど、それは思いつけなかったなぁ。Although I'm sorry, I couldn't think of that.~~