I can't pick up the weapon I just crafted

I crafted a weapon but can't collect it. I have no weapons in my storage either.

Where can I get it?

You stated that you do not have any weapons in your inventory; do you have outfits, junk, or pets in your inventory? If so, depending on whether you have reached full capacity, this may be the issue. When it comes to storage, all of your total items in each category (Weapons, Outfits, Junk, and Pets) combine for a sum total in your inventory (as opposed to separate inventories for each category), which is displayed at the bottom of the inventory screen. For example, in an inventory with a capacity of twenty, you could have zero weapons, twelve outfits, and eight pieces of junk. Such a case would prevent you from collecting additional weapons.

If you do have room in your inventory, there are other questions that might lead to finding the missing weapon:

When you discovered that the weapon was missing, had you just-then clicked to craft it, or was it at the end of the crafting timer when the green gun and frame appears over the workshop for collection? If the moment of discovery was when you had first clicked, you may simply need to wait for the timer to complete. However, if discovered at the end of the timer...

When clicking to collect the weapon, did the green gun and frame disappear or persist? If it disappeared and the weapon did not appear in your inventory, check to see if one of your current dwellers was accidentally assigned the weapon in question. I have experienced a case of a missing weapon a couple times due to lack of attentiveness: When in the Overseer's Office mission screen, sometimes there will be a weapon damage requirement that my most-suited dwellers are not equipped with. In this case I have sometimes equipped them with better weapons from the inventory or even from other dwellers without really knowing which weapon was being equipped. If none of your dwellers have the weapon, or if the green gun persisted...

Did you lose your network connection? If you lost your network connection, then it could be possible that the game did not recognize your interaction with it. If this is the case, simply reloading the game should allow you to collect the weapon, as the time to completion would still have expired.

Depending on the point of discovery (pre-green-gun, for instance), there is also the possibility of having accidentally cancelled crafting the weapon, or worse having accidentally sold the weapon. In either instance, you will not be able to retrieve said weapon, but instead will have to restart your crafting of this weapon.