How to explain a missing grade card in unofficial transcript for online graduate application?
If you don't apply, you don't get in. Simple. If you do, then there may be some difficulties, but many of them can probably be overcome in the eyes of most institutions. Given the current pandemic, people are going to have to be flexible. 

I'd suggest that you have someone, maybe the department head, provide an unofficial listing of your courses and grades for that semester. Perhaps they are allowed access to grades, but they can also contact your professors to verify the grades if necessary. Explaining the grading process at your university would also be useful in such a note. 

A note from you explaining the situation might be helpful, since the reason was medical and the delay is well explained. 

I can't guarantee that anyone would necessarily accept this, but suspect that most will, especially since they only need official transcripts later. 

But, again, you have little (almost nothing) to lose by making a good faith attempt and putting together the best record you can for admission. People will be looking for evidence that you will succeed. Provide that.