には無理がある【にはむりがある】

には無理がある says that a plan, an argument or an idea is strained: it asks for more than is realistic, or it doesn't quite hold together. It follows a noun, or a verb in the dictionary form, and には marks the thing the strain is in.

After a verb, the dictionary form + には means "for doing ~", so the sentence says the task is out of reach. The verb stays in the dictionary form when the sentence is in the past: はしる走るにはむり無理があった, not ×走ったには無理があった.

には無理がある or 無理だ

むり無理だ is a flat "impossible", "it can't be done". には無理がある finds a fault in the thing itself: something in the plan or the reasoning is forced. That makes it the one to use for an argument, an excuse or a theory.

Softening it

A degree word goes in front of 無理: すこし少し, ちょっと, かなり. The expression is at home in essays and reviews, and in conversation too, where ちょっと無理がある is a mild "that's a bit of a stretch" and には often shrinks to は.