Prior period expense

Prior period expenses are not allowed in current year untill and unless the expenses have crystalised during the year.

The prior period expense partaining to earlier years has been a matter of litigation whether such expense could be allowed as deduction during the current year .

if specific to your case, Bill is of 31st March, 2020 but received in May, 2020
if it's service, than if service is rendered in F.Y 2019-20 and invoice is also issued in that year, than that is prior period expenses and disallowed.
But if it's relating to Goods, invoice of last year, but goods received in current year than it's considered as current year expenses.

Sir please tell me the meaning of expenses crystalized during the current year?

Generally while calculating Inc from profit or gains from business or profession for companies we need to consider their choice of method of accounting which they adopted in Books of accounts. If they are following accrual sys, then it is allowed in the year of accrual. if cash system is followed it should not be allowed in the year of accrual.
But as per the prov of companies act 2013, companies should maintain their Books of accounts in accrual system. so , allowed as ded in year of accrual.
but remember that 10000 limit for single peron in a day rule.