I lift weights but I can't do a single pushup?

So, for the past about, 2 months or so I've been lifting weights on a regular basis. I have pretty good muscle memory since I used to be in color guard and it required quite a bit of upper body strength. Granted, between my upper and lower body, my lower body is stronger, more toned and defined.

For the past 2 weeks I've been doing Jillian Michael's 30 Day Shred, level 1, and there is a little weight lifting but I also lift weights beforehand.

Thing is, I can't do a pushup :/ It's a struggle to do even 1. For wall sits, I can do 50+ squats and lunges and sit in a wall sit for a couple of minutes, but my upper body sucks balls.

What can I do?
I lift weights but I can't do a single pushup?
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