You can't flash twrp with 8.8Those having problems with password,
Its asking for the encryption password. The default password is "default_password" (without the quotes) try that.
If it doesnt work, BEAWERE ALL YOU DATA WILL BE GONE. try format sd, format data (where you have to write YES) , format cache. This will delete encryption.
Last option and permanently solve: you should go back to previous version of rom. I guess its 188.8.131.52 - go back to stock oneplus recovery - update rom to latest version - flash twrp again.