Do you have two sturdy chairs or benches that you can place your barbell across? This would allow you to do horizontal rows. You must make sure they won’t tip while you’re pulling.
Do you have any exposed rafters in your basement or garage? Exposed iron beam in basement? An outside deck with any sort of height off the ground? With a higher deck you could by some cheap gymnastic rings to rig up something with the rings hanging off the outside about shoulder width apart and 7.5 feet off the ground.
Another option, if you’re going to look for a tree, you already blew the short rest intervals. You may as well just drive to your local school playground and use the bars there. Every school around me has something you can hang and chin from.
I’m just finishing up week 3 and I have a free standing pullup bar. Personally, if I didn’t have any way to hang I would substitute with power cleans or more bent over rows. I believe the key to this program is to use nothing but big, compound exercises with short rest intervals. The goal is to preserve muscle in your largest muscle groups while loosing fat via a decent caloric deficit.
Remember it’s only 28 days, so it’s just not a critical choice. But whatever you do choose for a substitution will work as long as it’s a systemically challenging movement and you use the RM and rest intervals specified.