erroring

Unfortunately you don't wrote anything about your system and detailed infos asbout your issue.

Please give us more informations about your system including the following:

• Adobe product and version number
• Operating system and version number
• The full text of any error message (s)
• What you were doing when the problem occurred
• Screenshots of the problem
• Computer hardware, such as CPU; GPU; the amount of RAM; etc

Please provide also a copy of your 'System Info' if it's possible. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

What you can do is to check if there's an .lock file besides your catalog file.

More informations you'll find here: The Lightroom catalog cannot be opened because another application already has it opened - lrcat.lock (adobe.com)