I started my game as England and formed Great Britain in 16th century. I assumed that Great Britain is a cultural union and would deem all cultures in British group to be accepted. However, Scottish remained unaccepted. Can someone shed some light on this? I have Common Sense expansion, if that is relevant.
Since patch 1.14 Cultural union isn't any longer tied to country tag, but can be claimed once your country reaches Empire level. If you don't have Common Sense DLC active, the limit is owning at least 1000 development.
Source: changelog for 1.14 (link)
Because Great Britain isn't an Empire, it's a kingdom level national title.
A cultural Union (now) requires you hold the Rank of Empire either through country decision (It'll specifically say it raises you to Empire Level, like Ottomans or Japan, or fixed rank Empire government-type like Ming's celestial empire) or having 1000 or more total development will let you simply declare yourself an empire, which grants a cultural union of what ever your primary national culture group is.