Tafsir of Az-Zukhruf 43:86

"And those whom they invoke besides Allah do not possess [the power of] intercession..."

Those whom they invoke besides Allah do not possess the power of intercession, contrary to their claim that these [idols/deities] are their intercessors before Allah.

"...except he who has testified to the truth..."

This refers to the one who testifies to the truth—which is the Oneness of Allah (Tawhid)—while knowing what he is testifying to with insight, certainty, and sincerity. He is the one who possesses [the right to] intercession. This is a munqati‘ (disjunctive) exception.

It is also permissible for it to be a muttasil (conjunctive) exception, because among those whom they invoke besides Allah are the angels.

Note: The verb "tad‘una" (you invoke) has been recited with the letter 'ta' (second person), and also with the 'ta' and a shaddah on the 'dal' (tadda‘una).

"And his saying, 'My Lord, indeed these are a people who do not believe.' So pardon them and say, 'Peace.' But they are going to know."