If you haven't used the Pledges module of FIMS before, it is possible that it was never initialized properly. Pledges must have numbers just like gifts and grants.
To set the next number:
Tools > System Utilities > Set Automatic Numbers
Choose Pledge Number
On the right, enter a number 1 larger that the value in the Last Promise ID Code used field.
If that field is blank, enter a 1.
Click Save and then Done.
You should be able to add a new pledge now.