You seem to have two copies of the Calendar application. Delete the second one. Never move or copy built-in applications.
If you still have problems after that, see below.
Please take each of the following steps that you haven't already tried, until the problem is resolved. Back up all databefore making any changes.
Step 1
From the Calendar menu bar, select
Calendar ▹ Preferences ▹ Accounts
Choose the affected account from the list on the left and uncheck the box marked
Enable this account
Then check the box again. Test.
Step 2
Make a note of the account settings, then delete the account and add it back with the same settings.
Step 3
If you can't add the account, the settings may be invalid. Verify them. How you do that depends on what kind of account it is. If it's a Google calendar and you've activated 2-step verification, you need to generate an application-specific password in your Google account and enter it in place of your old password.
Step 4
Quit Calendar. Triple-click anywhere in the line below on this page to select it:
~/Library/Calendars/Calendar Cache
Right-click or
control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a file named "Calendar Cache" selected.
Move the selected file to the Trash. There may be one or two other files in the same folder with names that begin in "Calendar Cache". If so, delete those files too.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (
command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
Step 5
Follow the instructions in this support article.
