Learn Swedish: Sortimentet innehåller många spännande senaper och vinägrar. - The assortment contains many exciting mustards and vinegars.

This sentence uses sortpluralis (plural of varieties). While you usually cannot count a substance like mustard or vinegar, you can pluralize the word when referring to different types, flavors, or portions of it. In this context, it means the assortment has many different types of mustards and vinegars.

Why does senap get an -er ending (senaper) while vinäger gets an -ar ending (vinägrar)?

When mass nouns are pluralized this way, they fall into standard noun declensions. En senap belongs to the group that takes -er. En vinäger takes -ar. Notice that because vinäger ends in an unstressed -er, it drops that e before adding the plural suffix (vinägrar), just like en cykel becomes cyklar.

Why doesn't the adjective spännande (exciting) take an -a ending to match the plural nouns?

Spännande is a present participle (a verb form ending in -ande or -ende acting as an adjective). Unlike regular adjectives, present participles are completely indeclinable in Swedish. They never change form, so it stays spännande regardless of the noun's gender or whether it is plural.