That's clear. Go to an instructor and ask him what is crafting. He will show you an explanation about levels and about restrictions.
|Level||Level Name||Amount of allowed skills for that level or a higher level|
|0 - 20||Apprentice||6|
|20 - 40||Craftsman||4|
|40 - 60||Expert||2|
|60 - 80||Master||1|
|80 - 100||Legend||1|
You already have Blacksmith and Carpentry at Expert level or above. That is the allowed two skills. Tailoring would be your 3rd skill, and that is not allowed ...
... UNLESS, you buy an [Crafting Skill Expert Expansion Ticket] in the Item Shop and use it.
But there are also instructors in Heffner Camp. In their early days, those ignored the restrictions (which were also different), but I think nowadays they also don't do it. They just safe you from doing a simple quest.
PS: The Germans have two nice guides about it:
There are two rules. One for crafting skills. One for collecting skills.
Side note: There is also the secret 3rd rule, which only applies to very, very few people which had some very special levels which were only allowed to an very old rule system, but not possible anymore with the current rules. Basically this 3rd rule is: For each level, the sum of collecting and crafting skills must not exceed the sum of the two rules.