Trying to complete the Salesforce Platform Basics, Get Started with Salesforce Platform.  
Customize the Salesforce Platform Challenge:
I added the new field under Contacts, Prequalified Amount. The field name was automatically created/assigned as "Prequalified_A__c".  The data field type is currency.  I tested it to make sure, but I'm getting this error.  

Challenge Not yet complete... here's what's wrong: 
The field 'Prequalified_Amount__c' either does not exists on the Contact object or it is not of type currency.

Thus, I manually edited the field name to "Prequalifed_Amount__c", but then  this error displays: 
Error: The custom field name you provided Prequalified_Amount__c on object Contact can only contain alphanumeric characters, must begin with a letter, cannot end with an underscore or contain two consecutive underscore characters, and must be unique across all Contact fields.

How do I resolve the first error?