If you uninstall, make sure the uninstall deleted everything in here:
C:\Program Files\ArcGIS\Pro
c:\Users\Administrator\AppData\Local\Microsoft\Windows\INetCache\ArcGISUpdate\ArcGISPro\ArcGISUpdate.exe
RegEdit Delete everything in:
HKEY_LOCAL_MACHINE\SOFTWARE\ESRI\ArcGISPro
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\RADAR\HeapLeakDetection\DiagnosedApplications\
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\bam\State\UserSettings\
HKEY_USERS\S-1-5-21-4090609309-3682410107-983408365-500\Software\Classes\Local Settings\Software\Microsoft\Windows\Shell\MuiCache
HKEY_USERS\S-1-5-21-4090609309-3682410107-983408365-500\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.aprx
most importantly delete here:
HKEY_USERS\S-1-5-21-4090609309-3682410107-983408365-500\Software\ESRI\ArcGIS Online For Pro\
^^ that's where all your portal logins are saved