Check In/ Out Not Working

[AdilTyabji]

Hi,

I've got a top level assembly that suddenly, as I was working on it, came up with this message "...iam is not checked out to you. Edits are not allowed on this file."

The project is set to (read only), Single User and Vault has not been set up or enabled.

I've turned on "Enable creation of legacy project types" in Application Options but this hasn't resolved the issue.

I've seen that some people have a Check In/ Out tab under the Model tab but this option doesn't pop up for me.

Does anyone have any suggestions please?

Thank you.

Adil