Resistance training performed with loads as low as 30% of one-repetition maximum can produce muscle hypertrophy that is comparable to that produced by higher loads when both are performed to muscular failure.

Study: Divergent Strength Gains but Similar Hypertrophy After Low-Load and High-Load Resistance Exercise Training in Trained Individuals: Many Roads Lead to Rome.

This study found that lifting light weights until you can't do another rep builds muscle just as well as lifting heavy weights—so if you can't lift heavy, going lighter but pushing yourself super hard works just fine.

Study: Muscle Failure Promotes Greater Muscle Hypertrophy in Low-Load but Not in High-Load Resistance Training

If you lift light weights but push yourself until you can't do another rep, you can build muscle just as well as someone lifting heavy weights — but only if you go all the way to exhaustion.

Study: Muscle hypertrophy and strength gains after resistance training with different volume matched loads: a systematic review and meta-analysis.

Even if you lift light weights, as long as you do enough total work (same number of sets and reps with the same total weight), your muscles grow just as much as if you lifted heavy weights.