Solve salesforce class and trigger issue

Hi, Do you know how to fix something with this error message?

There was an error converting the lead. Please resolve the following error and try again: OpportunityTrigger: execution of AfterInsert caused by: System.FinalException: Record is read-only Class.RelateOppToMonthlyRevenueTarget.getInfoFromRelatedOpps: line 34, column 1 Class.RelateOppToMonthlyRevenueTarget.: line 9, column 1 Trigger.OpportunityTrigger: line 3, column 1