Measure based on another measure

If I interpret what you want that 2nd measure to do, it's to go through each policy and multiply by the proportion of a year that the policy has remaining, as of March 31. You then want to sum up all of these amounts across all policies - but only for a subset of policies that prior to March 31 and which had not expired by March 31.

Try this:

M2 Unearned Premium =
SUMX (
    FILTER (
        Coverage,
        AND (
            Coverage[EFFECTIVE_DATE] <= DATE ( 2019, 3, 31 ),
            Coverage[EXPIRE_DATE] > DATE ( 2019, 3, 31 )
        )
    ),
    Coverage[ANNUAL_PREMIUM]
        * DIVIDE ( ( Coverage[EXPIRE_DATE] - DATE ( 2019, 3, 31 ) ), 365, 0 )
)