You can review games but only when Epic says you can.
Epic Games Store review system

User scores are a real mixed bag. You have people that will fairly rate games based on their honest opinions. These sorts are rather rare. You have users that will rate on an "all or nothing" scale where only the lowest and highest numbers are viable options to them. You have those that will rate a game highly because it's a meme. Then you have those that will participate in review bombing of a title because of a variety of reasons.

It's for that last reason that some storefronts, such as the Epic Games Store, have decided to take an alternate approach to how their games are reviewed by players. Epic just started to roll out user scores for the games offered through their storefront. Their solution won't be a wild west situation like Opencritic where literally anybody can put up a score regardless of game ownership. It also won't be like Steam's system where you need to at least own the game and with anti-review bomb systems in place.

Epic's solution, as detailed in an Epic Games Store update post, will offer random players a chance to review a game. These opportunities provided to players will pop up after a play session and only after the game has been played for a total of two hours or more. Players will be able to rate a game up to five stars. That seems familiar...

In addition, there will also be several questions that will pop up from time to time related to their most recent game session. These questions may be multiple choice or simple "Yes/No" questions. Some of these questions may ask if this is a good game to play with a group, or ask if the game has good boss battles, and so on.

Epic Games Store user polls

Here's how it works: Following a play session, random players will be offered the opportunity to score the game up to five stars. Over time, these scores will help populate the “Overall Rating” that will appear on the product’s store page. Because these requests are randomized, we won’t spam our players, and we probably won’t ask about every game or app used. This approach protects games from review bombing and ensures people assigning scores are actual players of the games.
Responses to these random polls are then used to create tags for the store pages. Would-be buyers can then use these tags to filter through the Epic Games Store for similar content. Epic will then use these tags and filters "to generate custom tag-based categories driven by our players that will appear on the Epic Games Store home page."

This review system seems a bit... odd. If you think a game is terrible, for whatever reason, you probably aren't going to be playing it for the two-hour requirement needed to review it and inform others. The way this system is set up seems to be intentionally designed to weed out a vast majority of negative reviews that could be made for real reasons and not just for review bombing. It also does not seem to offer any way to write in thoughts about why you scored a game the way you did in order to better inform others.

In addition, the questions posed in the polls that pop up offer very limited choices. Most of the questions and response options are skewed in such a way that it's near impossible to choose anything but "positive" leaning responses. If a game is ugly or has graphical issues, you can still only pick from choices such as "Beautiful, Realistic, Playful, or Unique" when asked to best describe a game's visuals. It makes sense when you remember that the polls are more for purposes of creating filters and lists for the store rather than asking for a player's honest opinion about a game or their recent play experience.