the problem is pretty simple: if we stipulate God's omniscience to mean God satisfies all (supposedly) ideal epistemic conditions for knowledge (infallible, justified, reliable, true belief with respect to all propositions & their relata) then it follows necessarily if God knows you will x then it's true necessarily *that* you will x, which implies there's no possible world in which the agent has the capacity to x over y, and if we cache out free will contra-causally (the capacity to x over y in any possible world ceteris paribus), then by definition if God exists then free will cannot exist. and if free will exists then God cannot exist. the same problem applies to Gods agency himself.
one response is to adopt a compatibilist view of free will (freedom consists in the agents capacity sefl-regulate & self-determine their actions despite having no capacity to do otherwise - which is consistent with determinism). the problem with that in this context is the problem of source-hood. in what sense is the agent the source of his actions if God is the ultimate source of all creation & the metaphysics of source-hood itself? that implies (compatibilist) free will is only possible if God doesn't exist - if there is no metaphysical grounding to source-hood other than the agent themselves. in which case you get an intractible dilemma: either (compatibilist) free will exists which entails God's non-existence, or, (compatibilist) free will exists and God exists but God is causally impotent (epiphenomenal), but if God is epiphenomenal then God cannot create anything, but God is a creator by definition, therefore, via reductio to contradiction, God cannot exist. the same problem of source-hood applies to God himself only worse because you get a boot-strapping problem: God's source-hood would have to logically precede itself, which is incoherent.
another response would be to adopt theological determinism. problem here is determinism as a general thesis with respect to agency is incoherent because it implies an eliminative ontological reduction from agency to mechanism. basically, if determinism qua determinism is true, then agency is reducible to disposition (like a rock), which voids agency altogether. since agency is intrinsically 1st personal and disposions are intrisically 3rd personal there can't be any reduction. this is a problem regardless of whether God exists or not & applies to God himself.
another response is open theism, which says God doesn't know the truth-value of future contingents. the future is genuinely open. number of problems with this too. 1st it contradicts Gods omniscience from the outset. 2nd it generates a boot-strapping/regress problem (Gods non-knowledge of future contingents are always indexicalized to his present knowledge which he can't have given either is omnitemporality or atemporality + he'd have to know that he knows ad infinitum).
another response is molinism. also fucked.
https://useofreason.wordpress.com/2017/03/03/molinism-and-trivial-counterfactuals/
https://useofreason.wordpress.com/2017/11/21/molinism-and-the-grounding-objection/
https://useofreason.wordpress.com/2018/08/07/molinism-and-the-grounding-objection-part-2/
also, saying God is atemporal (outside time) is irrelevant. the point is there is a logical incompatibility with respect to *creaturely freedom* if God is omniscient. the force of the argument marches on regardless of Gods ontological relation to time. more fundementally, tho, if God is atemporal then he can't have any agency, since agency presuposses temporality & causation. and he can't be omnitemporal either because that's incompatible with is immateriality.