Hey @leomarameo7,
the reason you get that error is that you forgot to use mi() for solar_z as a predictor.
bf(plankton_abun_z | mi() ~ 0 + Intercept + mi(solar_z) + mi(macrophy_abun_z) + water_volume_z + DOC_input_z) + gaussian()
instead of
bf(plankton_abun_z | mi() ~ 0 + Intercept + solar_z + mi(macrophy_abun_z) + water_volume_z + DOC_input_z) + gaussian()
and so on for the next two formulas.