I have the master sword, but when I go back to the lost forest to start the trials, I am not getting option to start.
Based on the comments above, you do not have the DLC content installed for Breath of the Wild. The Trial of the Sword is a challenge that was released within the Master Trials, the first DLC released from the Expansion Pass. The Expansion Pass can be purchased from the Nintendo store for ~$20 USD, or whichever your local currency is.
Once the DLC has been installed to your device, the Trial of the Sword will become available and you can place the Master Sword into the pedestal.