Petra Solutions is the team project name.
Deleting this will remove all your code, work items, builds, everything! And it is not recoverable.
If that is the case then, there is an option in the Web Access to do that now.
Or via the command line:
TFSDeleteProject /force /collection:https://mydomain.visualstudio.com/DefaultCollection "Petra Solutions"
If that is not the case, and you just want to delete the PetraERP_1 folder of code, you want the tf delete command line to delete the source code only (this is recoverable).
To use tf delete you need to have a working copy of the code checked out. You can do a non-recursive checkout with the tf get command in a Workspace with a mapping to that path.