MY Steps;
1- USB debug open(on chinese rom, i installed on MiFlash with chinese fastboot rom)
2- TWRP 3.2.3-0810 flash, open,
3- Make Format, say yes
4- install Beta rom (xiaomi.eu_multi_MI8SE_8.11.15_v10-9)
Hi folks. Same problem but ı solved yesterday. When you open twrp then you must format before install rom. Format it and say yes then install zip then its works