I watched an old Corona tutorial on YouTube and he uses Corona Portal MTL but I didn’t find it
Since introduction of adaptive environment sampler several years ago Corona portals became obsolete and were removed from Corona.
The issue with old tutorials is… that they are old and does not reflect current state of Corona. It is advisable to search more recent tutorials and other learning material because industry is never standing still and you always want to learn most recent things if you want to keep your skills sharp.
All the portal-related videos on the Corona YouTube channel have a note in the description that they are no longer used in Corona. To further reduce confusion though let me add “outdated” in the title, for those viewing methods that don’t show video descriptions 
If you wish to follow old tutorials, please use old Corona versions 
Download | My Chaos
On a serious note: it’s always best to use the newest version and follow the newest tutorials.
You can also find out knowledge base here: https://support.chaos.com/hc/en-us/sections/4526265417105-Corona-Knowledge-Base-
And the Documentation here: Chaos Docs