Do you have a multi-collection or a single-collection package?
If this is the only info.rkt and there’s not a (define collection ‘multi) in it it’s a single-collection.
If it’s a multi-collection package, is this info.rkt the info.rkt for the package or for the collection?
(the (define collection ‘multi) should be in the one for the package, and
the (define scribblings ….) should be in the one for the collection)
If it’s a single-collection package, the info.rkt is for both the package and the collection.
And did you put “manual.scrbl” in the scribblings when the file is “main.scrbl”?
And if you have all this correct and where it should be, but didn’t when you tried to install it for the first time,
you probably have to either uninstall and reinstall or run raco setup on the collection.