I tried to snap chalk lines on Schluter-DITRA before setting my tile, but the chalk rubs off the plastic easily while I am working. What can I do?

One option is to apply hair spray over the chalk lines. You don’t have to worry about the hair spray acting as a bond breaker because the thin-set mortar applied over DITRA is locked into the cut-back cavities on top of the membrane. Another option is to use an ink line instead of a chalk line.