There are three possible reasons why the fishing rod doesn’t fit in your backpack.
First, you might not have allocated enough space for it. Ensure you have cleared enough room in your inventory to accommodate the rod.
Second, the issue could be that you haven’t organized your inventory to have three slots arranged horizontally, which is necessary for most fishing rods.
Lastly, if you are trying to purchase a Monster Rod, this particular item requires four free slots that are aligned in a single horizontal line. Please check your backpack’s organization and make the necessary adjustments to fit the fishing rod.
If you find yourself lacking space, you can remove some of your resources. To do this, click on the items you wish to discard, then select the trash bin icon and confirm your choice.