Which versions do I need?

Bio Tech 2.1.x needs Factorio 2.1 and Bio Industries 3.0.1 or newer. For Factorio 2.0, use Bio Tech 2.0.x with Bio Industries 2.0.34.

The game won't start and the error mentions Bio Industries or nitrogen.

That comes from Bio Industries, not Bio Tech. It creates some of its fluids too late for a newer part of the game that checks recipes earlier. This should go away once Bio Industries is updated.

Can I add this to a save that is already running?

Yes, but sand ore only generates in newly explored areas. With Oarc, the extra sand patch at each spawn only appears in games started with Bio Tech installed.

Why is the first photosynthesis panel locked?

Only if Krastorio 2 Greenhouse + Wood Pyrolysis is installed. The panel then needs its greenhouse glass, so it unlocks together with it.

Can I change how much pollution the panels absorb?

Yes. The startup settings have a value for each of the three panel tiers.