Question; how many Champions do you have at Mastery Rank 7?
If the answer is anything less than twenty, you do not qualify to play Ranked Summoner's Rift, and if the answer is any less than twelve, you do not qualify to play Ranked at all.
Because, since you need at least twelve Champions to play Ranked Flex 3's, and at least twenty to play Ranked Solo/Duo and Flex 5's, it would only follow suit that, if you need Mastery Rank 7 to play a Champion in Ranked, you should also meet the other requirements with it.
In other words, you should have twelve Champions at MR7 to play Flex 3's, and twenty at MR7 to play Flex 5's and Solo/Duo.
If you somehow fit into those that now qualify, congratulations, enjoy your long queue times, because I'm certain, out of 140~ Champions, even with a pool of a million-some players, you're not going to have too awful many with 20 MR7's.
And if you don't fit into that prerequisite? Well, sucks to be you. You can't play Ranked. Get to grinding that Champion Mastery.
If you don't like the grind? Well, you asked for it.
See how bad things can get when you put an arbitrary MR7 prerequisite on Ranked?