List of steps to reproduce (step by step, including full links if applicable): 1

• Create new item, e.g. Q108582029
• Try to add jvwikisource as a link
• Failed
• Try using special page Special:SetSiteLink/Q108582029
• Failed: "Penanda situs "jvwikisource" tidak dikenal. GUnakan penanda situs yang sudah ada, misalnya "enwiki"" (The site ID "jvwikisource" is unknown. Please use an existing site ID, such as "enwiki". )
• Edit existing item, same failure

What happens?:
Since jvwikisource is a new project (August '21), I believe there's a missing step to add the project to Wikidata. Please make sure newly created project will be added to Wikidata immediately.

What should have happened instead?:
I should be able to link to jvwikisource