why i can't be a Bard ?

To add, as above you must have your Bard Soulstone equipped for your Archer to transform into a Bard.
If you did equip it at some point, but forgot to update your equipment set, then when you switched to your Summoner/Scholar job and then switched back to your Bard, the Bard soulstone would not auto-equip and cause you to not have access to the Bard only actions/skills.
Also, the equipment sets are stored on your local game client, so if you logged in from another computer or console and manually switched jobs via weapon slot swapping, the soulstone wont move over.

Stay archer my friend. The people are at the archer guild are a good lot. Going bard means hanging out with a hobo in the woods. Nobody wants to play with a class that got their abilities from a hobo in the woods.