You probably enter the hero and click on the book, right?
Try not to enter the hero, but enter the book through the menu on the right. Or just press "C". It should work!
Good luck!
You need to select a spell on the right where everything is displayed and cast it, then it will work))))
I had the same problems a long time ago;) ))))) I found it completely by chance:D :D!! I just decided to see what else is there besides the end of turn and the menu)))
Added 4 minutes later
more details... On the panel where there is the end of turn, you need to find "Cast Spell" or something like that, depending on what translation you have, there will open a regular book of spells for the selected hero, and then you choose a spell and that's it. In the case of teleport, you can choose where to jump, in the case of the Gate, when upgrading Earth, you can choose cities, and I almost don't use the rest)))
