Hi, follow below steps:
Step 1: fill the line details.
step 2: add vat bus posting group. select vat prod posting group and click on select from full list.
step 3: create new vat product posting group vat 21 (if this name is already exist then give some prefix )
step 4: select new added vat prod posting group and click on setup.
step 5: set the vat details.
step 6: select the the new created vat prod postin grp
Hope this helps
