There could be 3 reasons:
- Required node is not available. Please check node status at "Top > Service Director > SiteBuilder Manager > SiteBuilder Nodes". Please read the article if the node is not available.
- SiteBuilder plan is not synchronized with SiteBuilder node. Please check sync status at "Top > Service Director > SiteBuilder Manager > Plans > PLAN_NAME > Nodes"
- License limitation. Please check "Top > Action Log". If you get error like "A new host cannot be added as active due to the license key limits" then you should contact sales representative in order to increate your license limits.