Hi and welcome.
Before doing anything else, did you do the install as right click/run as admin? And once that is done, go to the U90ladder.exe program itself and ensure you have Run as Admin selected in right click/Properties/Compatibility.
If you did not do the install as above,
save your plc programs,
connect the plc and manually fully delete the matching com port driver in Device Manager,
completely uninstall U90 using the standard uninstaller,
then delete the Unitronics U90 Ladder folder if it remains,
reboot and start again, doing the install as first mentioned.
In theory the latest versions of the program do the admin stuff themselves, but sometimes it doesn't work.
Once you have done that, see how it goes and then we'll proceed from there.