Yeah installment is not accepted... And somewhere in this or the may supervisa thread I remember seeing someone getting rejected for this same issue.
The IRCC site states:
This medical insurance needs to
be paid in full, or in instalments with a deposit (quotes aren’t accepted)
be valid for at least 1 year from the date you’ll enter Canada
cover your health care, hospitalization and repatriation
provide for at least $100,000 emergency coverage
You can refer:
https://www.canada.ca/en/immigratio...arent-grandparent-super-visa/eligibility.html