2 things.
First I'll address executing your command "Now"
Terminal -> Shell -> New Command...
...enter the command you want to run...
Second, trying to address why Terminal is not giving you a working window.
You most likely have a broken shell initialization script.
Since you are posting in the Mojave forums, I'm going to assume you have Mojave.
Open a Finder window to your home directory
Command-Shift-period (this will show all the hidden files in your home directory)
You are looking for any file that has a leading period and one of these names (note, these file may or may not exist):
- .bash_profile
- .bash_login
- .bashrc
- .profile
If you find any of these files, rename it to
- saved.bash_profile
- saved.bash_login
- saved.bashrc
- saved.profile
Command-Shift-period (to allow the hidden files to disappear again)
Quit Terminal
Re-launch Terminal.
Is it working properly?
If it is not working properly, then
Quit Terminal
Finder -> Go -> Go to folder... -> ~/Library/Preferences (that ~ character is a tilda and is generally found on the upper left side of your keyboard)
Drag the com.apple.Terminal.plist file to the desktop
Re-launch Terminal
Do you get a working Terminal window now?
If not, please post any errors and diagnostics you saw (copy and past, or screenshot so we can see the exact error)