Learn Croatian: Večera je bila odlična.

It is a very common mix-up! Večera (ending in -a) is the meal: "dinner". Večer (ending in a consonant) is the time of day: "evening". So, you eat večera during the večer.

Could I change the word order and say Večera bila je odlična?

No, that would be grammatically incorrect. The word je is an enclitic, which means it is a "weak" word without its own stress. Enclitics must almost always take the strict second position in a sentence or clause. Here, it naturally falls right after the first word, Večera.

If bila already means "was" for a feminine subject, why do we also need the helper verb je?

In Croatian, the standard past tense (the perfect tense) is a compound tense that requires two pieces to function properly. You need the present tense of the helper verb biti (which is je for the third-person singular) plus the past action word, called the l-participle (bila). Dropping the je makes the sentence sound broken, much like saying "Dinner been excellent" in English.

Why does the adjective odlična end in -a here instead of being the dictionary form odličan?