"CoepI" is what is unkindly but accurately known as a "defective
verb". Most verbs can be used in most or all tenses, but some verbs are
only used in a few tenses. We don't know whether they ever had a full
set of forms or not (the person who coined the name "defective"
apparently assumed that they didn't), but, if they did, we have no way
of knowing what those forms were.
It's not used in the present system (imperfect-present-future), only
in the perfect system (pluperfect-perfect-future perfect). Most verbs
have a first principal part which is the first person singular of the
present tense. Likewise, then, its first principal part, "coepI", is the
first person singular of the perfect tense. Most verbs use the present
infinitive as the second principal part. It uses the perfect infinitive.
There is no need to give its third principal part (usually first person
singular of the perfect tense) since we'd just be repeating its first
principal part. And its fourth principal part (perfect passive
participle) is the same as any other verb's.