Block layout and custom block library tabs missing

That is intentional. In 10.1.x that menu item is now on the Content menu.

The menu item is still where it was under Structure, but the URL has changed to match admin/content. I think the update hook might have been overlooked for existing sites?

Hello, I have the same problem. It doesn't appear in the menu under "Block layout", I have to do "place block" then "Add content block" to add a new block but I don't have access to the library. I'm in 10.2.4

i have same problem and i find the way on youtube ,
to add a new block with custom block type : go to Content -> Blocks tab -> add content block button and choose it

admin/content/block