Why does Odysseus want to avoid the island of the sun? What was he warned not to do there?

Odysseus does not want to go to Thrinacia (modern-day Sicily) because he has been explicitly warned by both Circe and Tiresias not to. Odysseus was sent by Circe down to the underworld, and that is where he encountered the blind seer. As Tiresias is a prophet, it is especially important for Odysseus to heed his warnings about Thrinacia.

Unfortunately for Odysseus, he gives in to the constant pleading of one of his crew members, Eurylochus,...

Odysseus does not want to go to Thrinacia (modern-day Sicily) because he has been explicitly warned by both Circe and Tiresias not to. Odysseus was sent by Circe down to the underworld, and that is where he encountered the blind seer. As Tiresias is a prophet, it is especially important for Odysseus to heed his warnings about Thrinacia.

Unfortunately for Odysseus, he gives in to the constant pleading of one of his crew members, Eurylochus, and decides to rest a while upon the island. But Odysseus explicitly warns his men not to kill any of the cattle on Thrinacia, as they belong to the sun god Helios. Eurylochus, however, has other ideas and incites the other men to disobey Odysseus's express orders and slaughter Helios's cattle.

A suitably enraged Helios pleads with mighty Zeus to punish these presumptuous mortals for their disobedience. The father of the gods complies and unleashes a particularly vicious storm upon Odysseus and his crew. Their ship is completely destroyed, and only Odysseus survives the terrible wrath of Zeus.