Prior period expense

Please inform what type of expenses ?.If Expense were of F.Y 2019-20 then you can claim Input Tax credit up to the date of September month GSTR3B return of 2020 .If expenses are of block credit then you can not claim the same .
In Income Tax perior period Expense will not allowed as deduction . please refer IND AS 8 .

It is legal fees paid to an advocate

As per notification 13/2017 dated 28th June 2017 , supply of legal services , RCM applicable to GST registered company . If you have accounted in the books in current year and paid GST on RCM , then you can claim up to September 2020month 3B return date , if expenses pertains to F.Y 19-20 .
In Income Tax you may not get deduction as perior period expense.