You are correct that under most circumstances, you don't use an adjective when there is no noun. Adjectives modify nouns, so if you're not modifying a noun, you almost never use an adjective.

There are some circumstances when a noun is implied. In that case, you can use an adjective. A common example of this is "la angla" which implies "la angla lingvo."

Waiting for clarification on the "sekva" example you mentioned. It's difficult to comment on it if we don't know what it is.

In rare circumstances, and adjective and be used to modify a whole phrase if doing so would resolve an ambiguous reading. I'm still looking for the reference and will update this answer when I find it.

You are correct that under most circumstances, you don't use an adjective when there is no noun. Adjectives modify nouns, so if you're not modifying a noun, you almost never use an adjective.

There are some circumstances when a noun is implied. In that case, you can use an adjective. A common example of this is "la angla" which implies "la angla lingvo."

Looking at the "sekva" button on Lernu.net, I would say that this usage is OK because the noun is strongly implied. It means sekva paĝo or sekva alineo.

In rare circumstances, and adjective and be used to modify a whole phrase if doing so would resolve an ambiguous reading. I'm still looking for the reference and will update this answer when I find it.