Many of salesforce programmer face problem while converting the Lead into Accounts, Contact, Opportunities and followup tasks.
Normally the reason is custom field created by you in any of above which is mandatory and not present in Lead.
For example in Below screen:
The error is caused because of field Phone extention in Account which is mandatory.
So, to resolve this kind of error, following approach can be used.
Determine the mandatory fields which are present in Opportunity, Account and contacts.
If those fields are not present in the Lead or not mapped in the lead than create those fields in Lead and through field map functionality, map those fields with Account, opportunity whatever required.