My guess would be that this has happened at the point where Web installer has flashed the e-Recovery image but you now have to very importantly get the phone into the new e-Recovery in order to “break” the Manufacturer’s version of Android.
If you do a standard “Reboot into recovery” you will simply boot into the pre existing Manufacturer’s version of Android Recovery.
The trick is to avoid the phone from doing a standard shutdown and restart cycle. Several contibutors explain it in different words in this thread Problems installing /e/ OS on Oneplus 3 - can't boot into the /e/ recovery mode.
Not all OnePlus use exactly the same technique, your phone may have a menu. Still the same thing, you want to trick the phone into booting to your new recovery, not the old one – booting to the old one wipes, destroys, e-Recovery so you have to repeat the flash of e-Recovery and try again.
One thing to do is to write any question or feedback in your own language, then copy and paste a DeepL Translate: The world's most accurate translator translation to English below.