If you are using keyboard and mouse to play Roblox on the Xbox One, MorkyMork1991 stated you press the "/" key to bring up chat when you are in game. This should open a text box for you to type once you have already loaded into the game. You need to check your in game settings to make sure chat is enabled.
Steps
You can view your online Xbox friends from the Friends tab in Xbox and create a party to play with them. To view all of your friends, or accept/reject friend requests, you can do so by logging into your Roblox account on another platform such as PC or Mobile.
Provided you meet the above condition, simply follow the below steps to enable the Spatial feature:
To see the default emotes, hit the “/” key to chat and try typing in the following commands:
One of the many great ways to communicate with your fellow players is to use the in-experience chat feature. To start chatting, press the / key on your keyboard. This will open up the chat window, if enabled by the developer, and a chat-bar where you can type what you would like to say.
The second reason why the can't chat in Roblox error happens is related to the fact that the developers added a default system of settings. As a result, the contact settings are set to “No one” in the privacy section. Changing this setting to everyone will fix the can't chat in Roblox error.
Changing Privacy and Chat SettingsLog into the account. (If you can't log in, try these steps for recovering your password.)Go to account Settings. Browser - find the gear-icon located at the upper-right corner of the site. ... Select the Privacy tab.Adjust the Contact Settings and Other Settings.
Right-click the little speaker icon on the far right side of your taskbar, then click Open Sound Settings. Under Output, make sure you choose your speaker or the device you wish to use. You can double-check by clicking App volume and device preferences and setting the output device and volume for Roblox.