Monster Hunter: World if full of many ways to upgrade, but the most important thing you need to learn about is the upgrade system. Upgrading your weapons can be a little confusing at times, but once you learn how to do specific things, it will be very easy. Here is why I can't upgrade my weapon in Monster Hunter: World.
You don't have enough Zenny
This reason is only for the newer players of the game because you don't get that much Zenny for each mission you complete. If you ever find yourself running out of coin, then make sure to go and complete a tone of the optional quests so you can get that Zenny for upgrades. If you can, play it by yourself so you can get all the money from the quest and not split it with other people.
You haven't found the needed materials yet
Most of the time, you will find yourself looking at the '???' sign under the weapon you want. This means that you either need to find the material you need to see how many of that material you need, or you need to fight the next creature that can unlock all the materials for you. If you find yourself in this position, you probably have the best weapon you can get at that current time.
You don't have the next tier in your tree unlocked yet
The final reason you might not be able to upgrade is you have not received the first weapon from a specific tree. You will notice that the weapons in certain trees don't have anything under them, that is because you have not progressed enough to start a certain tree. You need to progress a little bit more into the story if you haven't or if you don't own Iceborne, you will be locked at Rarity 8 weapons. Just progress as far as you can and you can upgrade your weapons.
