We are running Sitecore 9.1, will Sitecore JSS 15 be compatible with this version?
Sitecore JSS 15 is not compatible with Sitecore 9.1.
You can use, Sitecore JSS 15 with Sitecore 10 or you can use Sitecore JSS 11 with Sitecore 9.1.
You can check the complete compatible table here. https://kb.sitecore.net/articles/541788