Error: Can't Remove a Team Member Assigned to an Opportunity Split

This can sometimes happen when the split assigned to the user is for some reason no longer visible from the user interface. Follow these steps to fix the issue

1- Log into your organization
2- Go to https://workbench.developerforce.com/login.php and click Login With Salesforce
3- Go to the Query tab and click on SOQL Query
4- Enter the following SOQL query in the query area| select the object: Opportunity split - Replace the Opportunity Id with the Id of the Opportunity in question (You can get this from the Opportunity URL)

Select Id, OpportunityId From OpportunitySplit Where OpportunityId = '0062000000a5Ot9'

5- The query will give you a list of the splits on the Opportunity. Most likely, you will see more rows than the actual number of splits in the Opportunity, and this is because one of those splits is not visible in the user interface.
6- Hover over the Id column and click View in Salesforce to identify the Split that is not visible in the interface
7- When you identify the split, delete it

After this, you should be able to delete the Opportunity Team Member.