Had this issue on my 16 Pro Max, had to put the phone into DFU mode and restore it to 18.5. Haven’t tried to install 26.0 beta again yet. I recommend trying these steps to enter DFU mode if you haven’t tried them yet. It was the only thing that worked for me: “First, quick-press the Volume Up button, then quick-press the Volume Down button. Then, press and hold the Side button until the screen goes black. As soon as the screen goes black, press and hold the Volume Down button for 5 seconds. After 5 seconds, release the Side button while continuing to hold the Volume Down button for an additional 10 seconds.”