GST Adjustment / GST 15% on Imports is normally a 100% tax rate code.
If it has not been used, on the Settings → Tax code screen the transactions column shows a dash “-”
On your screen shot I can see neither a dash or a number of transactions.
What account does your GST Adjustment / GST 15% on Imports tax code use.
Edit
On further testing Manager replaces the dash with a blank if the tax code has not been used and is inactive. Which means you do not have a duplicate tax code problem or tax codes using a mixture or accounts.
So you will need to use the drill down @Tut describes is likely to answer the question.