If we are creating a marketing campaign for including in the lead source drop-down in the customer, prospect, or sales order record the ID will be in front of the lead source for resolving the issue.
Go to the Auto-generated Numbers page:
Setup> Company> Auto Generated Numbers>
Click on the CRM subtab and disable or uncheck the corresponding to marketing campaigns
Go To the Marketing campaign creation page.
Create a marketing campaign without providing an ID.
Save the record