There is no need to decline the ITA. Instead you can go ahead and submit your application. When you submit your application, you can include a LoE, in which you can explain the issue, and also submit the documents pertaining to your any upcoming medicals that you have reserved, or screenshots of the available appointments etc. i.e. anything to support your case.
As per IRCC's COVID processing:
Applications with missing medical exams
If an officer receives an application with a missing Immigration Medical Exam (IME), the current 30-day submission period is a reasonable timeframe to expect the client to undergo the exam. If a physician is not available in the client’s area, it can be confirmed by the appropriate
Regional Medical Office (RMO) and the officer can make a reasonable decision as to whether to leave the application open or not. Officers may continue to request reassessments for expired IMEs where available using existing guidance.
https://www.canada.ca/en/immigratio...ce-delivery/coronavirus/general-measures.html