No, an upfront medical is mandatory as a part of your complete application, R10.
If you cannot provide an upfront medical, you can explain the reason in the LoE, and submit your application. Currently IRCC is not rejecting any application with missing documents that have an explanation.
The policy is:
Application intake
Intake of new permanent residence applications will continue. F
iles that are incomplete due to unavailable documents will be retained in the system and reviewed in 90 days.
New, complete permanent residence applications under section 10 of the Immigration and Refugee Protection Regulations (IRPR) will be processed as per normal procedures while taking into consideration the additional processing guidance outlined in these instructions.
If a new application is missing supporting documentation (associated fees are required), the applicant should include an explanation with their application that they are affected by the service disruptions as a result of the novel coronavirus. The application may then be promoted and reviewed in 90 days.
New applications should be promoted in the order they were received, and the org ID should be associated with them when the applicant self-identifies. If the application is still incomplete in 90 days, officers should request the missing documents with an additional 90-day deadline.
Applications found to be incomplete with no explanation provided, or for reasons unrelated to the disruption of services associated with impacts of the novel coronavirus, may be rejected as per section R10, and all fees associated with the application should be refunded to the applicant. The reason for rejection should be unrelated to the disruption of services.
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/service-delivery/coronavirus/permanent-residence.html