Raise the action or use heavier strings.

Fret buzz in lower tunings is normal on a budget acoustic because string tension drops, and the neck can bow or the strings slap against frets. You have two cheap fixes.

First, tighten the truss rod slightly (1/8 turn at a time) to add a bit more relief. This helps if the buzz is in the middle of the neck. Second, if the buzz is near the body, shave a little off the bottom of the saddle (or file the nut slots deeper if they’re too high). But go slow—you can’t add material back.

Alternatively, go up a string gauge. Lighter strings buzz more at low tension. Try 12-53 instead of 11-52. If you’re tuning way down (like open D or C#), consider a set of 13s. They’ll feel stiffer but hold tension better.

You won’t get a perfect, buzz-free setup on a $150 guitar. Some buzz under heavy strumming is acceptable. The audience won’t hear it.