You put your wallet with thousands of dollars into your secure container, and that caused it to weigh too much and you to become encumbered.
Here is why:
The weight of money in the game is set to 1 gram per dollar, as a U.S. bill weighs approximately 1 gram. While higher denominations like the $100 bill exist and weigh the same despite carrying more value, we’ve applied a consistent weight to all cash for simplicity. Introducing different denominations, including coins, would add unnecessary complexity. Additionally, we want money to function as an "item" rather than just a number, adding a layer of strategy when extracting large amounts.
How to resolve it?
Place your wallet in your locker. Whether you collect money in a wallet or a secure container, it will take up one inventory slot in both cases.