Before you head off to Golem Island Solo, it’s accessible after completing the “No Way Out” mission and talking to Jericho in Erewhon.
Those 2 tasks are required FIRST, THEN, head to the bivouac on the south OUTSIDE of the island to do Golem Island Solo.
From there, you will have to boat into Golem and climb a couple of ladders to finish off the unlocking of the island.
If you need a visual how-to, watch the video below.
