This seems like you don't have mate shell support for TextMate editor in your terminal.
You need to install mate shell command by:
export EDITOR="/usr/local/bin/mate -w"
The path above mentioned is according to my machine.
Alternate, you can install shell support from your TextMate editor by:
Preferences > Terminal > Install
After that, you are good to go
EDITOR="mate --wait" bin/rails credentials:edit