The claim: "2:48 says intercession will not be accepted; but 2:255 and other verses speak of intercession by permission. A contradiction."
The two statements deny / affirm different things
- 2:48 (and 2:123) denies an independent / guaranteed intercession — it dismantles the polytheists' hope that "our mediators will save us": "no intercession will be accepted from it, nor ransom."
- 2:255, 20:109 and 53:26 carve out an exception for intercession bound to God's permission and pleasure: "Who can intercede with Him except by His leave?" (2:255); "intercession will not avail except for one whom the Most Merciful permits." (20:109)
So what is denied is autonomous, guaranteed mediation; what is accepted is intercession only by God's leave. These are different propositions; there is no contradiction.
An honest limit
This harmony is a linguistic-logical reading, resting on the verses' own clauses ("except by His leave" / "whom He is pleased with"). While the scope of intercession (its extent, for whom) is a debate between schools, the claim "2:48 contradicts 2:255" loses its force once the clauses are seen.
Source: Qur'anic verses (M. Okuyan meal). Presented soberly and respectfully, with a text/interpretation distinction.