I paid for Didi Chuxing in cash, but kept texting me saying that I used Didi taxi and didn't pay.

For cash payment orders, the software prompts that there is no payment, please contact customer service. The software does not support cash payment. Please pay the order online later. Otherwise, it will easily lead to personal economic losses.

It seems that Didi payment is really an online transaction, and there is no need to give cash directly (once on an express train, the driver said not to say cash). I suggest you call Didi's complaint phone to reflect your situation, hoping to help you.