In order to successfully activate an artifact, you will need to have all three scrolls combined into a single volume, as well as the completed Encounter credit.
To combine your scrolls, you can place one on your quickbar and "use" it like you would a potion. The default "use" key is E, but if that isn't working for you, you can check it by typing /keyboard. Make sure you do not have any duplicates of the same scroll pages in your bags when you combine them as all the scrolls will be consumed and any duplicates lost.
You can obtain encounter credit one of two ways. You can either complete the physical encounter itself in the game, or you can purchase the credit from the NPC, "Aphaestia" in the Hall of Heroes. In order to use the purchased Encounter Credit, once purchased, you would only need to give the item back to the NPC, and credit will be granted.
Before attempting to activate your Artifact, please make sure that your quest log is not full, as the journey of recieving a powerful Artifact will show up in your journal.
If after all of these steps you are still unable to activate your Artifact, submit an appeal to In-Game support through /appeal, and they will be able to assist you further.