The error message is probably due to Tableau requiring the input to TOTAL() function to be an aggregate. Try the formula below to see if it solves the problem.
SUM([Quantity From Merchant to Installer OIL]) / TOTAL(SUM([Quantity From Merchant to Installer OIL]))
Hope this helps.
Thanks Ossai - I found the solution more simple in the end, just using the percentage of column under analysis