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Wait a minute I think you missed something, yesterday when I read your message in Discord I thought you were unable to install RVM which were producing that error. But this question clears my confusion. You were executing `rvm install "ruby-2.6.6"` but that's not what we used to do. 

I had written the same answer in another question of "mine". [here it is](https://linux.codidact.com/posts/282548/282557#answer-282557) (I had mentioned it in Discord either) (I don't have any idea if it's really good to link my old answer which account I had deleted, if it's bad purpose then you can edit or suggest edit). You should have execute

```bash
rvm install 2.6.6    #this will install 2.6.6 version of Ruby
rvm use 2.6.6        #you must execute it otherwise you will keep using earlier version (whichever you had in your system)
```

Sometimes, they don't work without being super user. So I used to use `sudo` but it sometimes doesn't work also. For that reason you should execute

```bash
/bin/bash --login       #I don't have idea, if the command really works in Mac
```

`/bin/bash --login` is nearly related to `su -`.