This message appears when the auto-numbering for creating accounts in AFAS must be configured differently. Depending on your settings, a number range is assigned to purchase accounts and sales accounts. This range must be adjusted within the 'borders' mentioned in the error message:
Lower border = number (lower limit)
Upper border = number (upper limit)
To change this, go in AFAS to Financial / Management / Settings and open the tab General, then adjust the lower and upper limits.
Also check in AFAS via General / Setup / Number settings whether the auto-numbering is correctly configured for 'Purchase relation' and/or 'Sales relation'.