The policy number reported to you by the Marketplace may be longer than the 15 characters allowed in the TaxAct® program entry field. However, only the last 15 characters are needed when filing your return.
The IRS Instructions for Form 1095-A Health Insurance Marketplace Statement, Line 2 state:
Enter the number the Marketplace assigned to the policy. If the policy number is greater than 15 characters, enter only the last 15 characters.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.