If you are the administrator, you can go to the solutions page and import a different solution.

Hi,

The error message is suggesting that something is wrong with your sitemap. Have you changed it recently?

Could you please see if the following link [your organization]/tools/systemcustomization/systemcustomization.aspx# works and check your sitemap?

Additionally, you can export the site map from a working instance and import it in the affected org.

Thanks