AVIVA only accept contributions on a calendar monthly basis. You should ensure payroll is set up to send pension contributions on this basis. Some payroll software can "roll-up" weekly, fortnightly or lunar contributions in to monthly submissions. Check with your software provider if you're not sure.
Premium users of pensionsync may find that Automatic Date Matching will help avoid errors where slight variations on these settings are used (e.g. payroll uses Tax Periods).