Magi comes in many shapes, and you can only insert a Magi that shares the shape of the slot in your equipment.
If the shape is not compatible with your current equipment, you can do either of the following:
Find another equipment that has a slot compatible with your Magi.
Find a different Magi that is compatible with your current equipment.
Also, as you play the game, you'll soon discover that some Magi are only for armor and some are exclusively for weapons.
If you still can't insert your Magi to the slot of your equipment even if their shapes are the same, chat with us (icon at top-right corner of support screen) or send Customer Support a screenshot of the Magi and the slot of the equipment, and we'll find you a fix.