Shouldn't use two extreme gears?
Using the smallest chainring with the smallest sprocket or the largest chainring with the largest sprocket should indeed be avoided. It’s called “cross chaining”. It’s less efficient and causes slightly increased wear and noise. On the small-small combination it can cause chain slap (because the chain will be almost slack) and on the large-large combination the chain is almost taught, so getting something like a twig caught in your chain can easily cause the rear derailleur to snap.
However cross-chaining is not very bad and it should work just fine. Just avoid it if you can.
Of course if you only have a single chainring it can’t be avoided and you should just use the gears when you need them.