In general, a due diligence clause is a clause that allows the buyer to cancel the sales contract without being liable for damages if, after concluding the sales contract, a detailed investigation of the property (due diligence), such as building defects or soil contamination, reveals some problem that is unacceptable to the buyer.
Since due diligence may take a considerable amount of time, as a seller, you will continue to have the risk that the contract will be terminated in the future, even after the purchase agreement has been signed. For this reason, sellers often disallow this covenant clause.