The investor password on the Metatrader platform allow for read-only access to your account. Trades cannot be placed or modified in investor mode.
When adding accounts to Social Trader Tools, Master/Copy from accounts can be added using the investor password as we only require the ability to read trades happening on the account.
Slave/Send to accounts require your full Metatrader credentials to open, modify and close trades being copied from your Master/Copy from account.
Adding your Slave/Send to account with investor credentials will lead to your trades not being copied, most commonly described in the copier log as error 133.