You receive the following message when trying to insert a record between existing records:
The Autosplitkey facility cannot find a key between the current and previous record
NAV allows you to insert between two existing lines up to 14 times. This is because NAV line numbering is in increments of 10,000, so when you insert a line between line 10,000 and 20,000 it creates line 15,000. As this number cannot be a decimal, it will get to the point where it cannot create a new number, this being the 15th line.
To get round this error add the new line to the bottom of the Order/Invoice etc