The API key does not work

The terminal rejects the key when adding it, or the key worked and stopped. Go through the list from top to bottom.

Check in order

Copied in full. The key and secret without cut-off characters. The terminal removes spaces at the edges itself.
Permissions. On the exchange the key has read and trade enabled. Withdrawal is not needed. Otherwise “The API key has insufficient permissions. Enable Read and Trade.” Details: Creating API keys and required permissions.
IP restriction. If the exchange has an address whitelist enabled, your current address must be in it. Message: “IP address is not in the API key whitelist”. The address changes when switching networks or VPN.
Passphrase. OKX, Bitget and KuCoin need the phrase set when the key was created. It cannot be viewed on the exchange; if in doubt, create a new key.
Real account. A key created in the exchange’s demo mode does not fit. Demo in the terminal exists only for Binance and is enabled with a separate switch.
Unified account. For Bybit and OKX the account must be switched to unified account mode, otherwise the balance check fails. KuCoin is the opposite: a regular (classic) account is required, the Unified Trading Account mode is not supported.
Computer clock. A difference from the exchange of more than 30 seconds gives an error. Enable automatic time synchronization in Windows.
Expiry. On some exchanges a key without an IP binding is valid for a limited time. Symptom: it worked, then “Invalid API keys”. Create a new one.

On an order “…: authorization failed — … Check the API key in the connection settings” appears: the key was deleted, expired or the IP changed. Create a new key and click Edit in the connection card.
After changing the computer or reinstalling Windows the connections have to be added again: the keys are stored only on that computer.

The full list of connection messages with explanations and exchange codes: Common connection errors.