Alter Aeon FAQ (Frequently Asked Questions)

FAQ 6.3 - What do I do when it says "You can't carry that much weight"?

You can either drop a few of the things you are carrying (or empty one
of your bags), or you can increase your strength.  If you are too weak,
you won't be able to carry a lot of equipment and extra weapons or
supplies.

One thing to look out for are monsters that may sap your strength.
These are typically undead or ethereal/ghostly creatures, though certain
types of poisons can also make you weaker for a while.  Most of these
effects wear off over time, but they can make fighting difficult if
you become too weak to wield your weapons.