1. TWINT test mode in the Payrexx backend
In your Payrexx account under "Payment provider", "TWINT", the mode can be changed to test.
2. contact TWINT
The test mode of TWINT is unfortunately not only depending on the setting in Payrexx, but also requires an account at https://portal-pat.twint.ch/partner/. In addition, you have to contact TWINT Support to get the test app sent to you by TWINT. Only then can you test TWINT.
You can also test TWINT live with small amounts instead. This results in much less effort.