A newly rolled out CS:GO update removes your capability to access Prime matchmaking for free. In the past, you could get Prime Status by collecting XP until you reach level 21 and associating a phone number with your account. Now, the only way to get the status — and, hence, the option to be matched with other Prime players — is to pay $15 for the upgrade.
You can no longer earn XP if you don't have Prime status, and you can't earn Ranks, drops or skill groups, as well. Based on people's feedback, it looks like you can keep your status if you've already reached it by taking the free path. However, if you were unable to reach level 21 before the update, you only have until June 17th to purchase the upgrade to reinstate the XP and Skill Groups you've already earned. By implementing those changes, Valve may have increased your chances of getting matched with other players serious enough about the game to pay $15 for it.
In its announcement, Valve explained:
"CS:GO went free to play over three years ago and is still going strong...Along with all the gameplay that we made available for free, new players had access to drops, Ranks, Skill Groups, and a free path to Prime matchmaking. Unfortunately, over time, those benefits have become an incentive for bad actors to hurt the experience of both new and existing players."
If you choose not to pay for the upgrade, you won't be able to participate in Ranked matches anymore. Valve has introduced Unranked matches for Competitive, Wingman and Danger Zone game modes, though, which anybody on CS:GO can access. It has removed Scrimmage from Competitive matchmaking and replaced it with Uranked that pairs people up from a pool of Non-Prime and Prime players. According to the update's patch notes, Unranked still uses skill-based matchmaking, but it doesn't affect Skill Group and have no Skill Group party restrictions.