The verb is is and the complement of is is myriad - which is a singular noun, not combinations.
You could not use an article if the myriad was plural
Myriad would have to have an s on the end of it if it were plural.
Combinations is part of the prepositional phrase of possible combinations which is modifying myriad. That doesn't make myriad plural. Prepositional phrases don't turn nouns into plural nouns
Since myriad already means many of, saying myriads means multiple instances of many of X which is probably not what is meant.
So, I was saying that either of two below would be valid:
There is a myriad of possible combinations
There are myriad possible combinations
But ... @StoneyB brings up a very interesting and good point in the comment about "oblique" subjects in cases where it is described by of X.
In particular, from the question cited in the comment, this:
Lots of workoblique is left to be done.
would sound wrong if said Lots of work are left to be done. It looks like lots (plural) is the subject of the sentence but it doesn't work with a plural verb.
Though the equivalent structure with the example sentence's words does not work with myriad because combinations is countable (and you have to use myriad with a countable noun because it means many of, not much).
Myriads of combinations is possible (sounds off to me)
In any event, using one of the first two sentences above is definitely 100% correct, but I am not sure whether "There are a myriad of possible combinations" is 100% incorrect.