Failed to create dir: \?\Larian Studios\Baldur's Gate 3

Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the My Documents folder?

Windows Defender's Controlled Folder Access feature has caused this error. An exception for the game can be made in Start > Settings > Updates & Security > Windows Security > Virus & Threat Protection. Then under Virus & Threat Protection settings, select Manage settings, and under Controlled folder access, select Manage Controlled folder access. Alternatively, the feature itself can be disabled.

BitDefender has also blocked the game from being able to create files and folders which has caused similar errors, generally with no question mark or non-path slashes. In that case, including both the game executables and Larian Studios My Documents folder in BitDefender's trusted app and directory sections should fix the problem . You can do this by following this path: Advance Threat Defense > Ransomware Protection / Safe Files > Application Access.

Avast's Ransomware Protection has caused problems by blocking the game's access to the My Documents folder. One report found that it was the launcher that was the issue and adding LariLauncher.exe to the trusted apps list fixed the problem. Discussed here. 

AVG has caused similar issues and in one case quarantined the launcher. Discussed here.

Try right clicking your My Documents folder and select Properties, switch to the Locations tab and click the Restore Default button. 

Try exiting out of the Steam client, if applicable, and starting the game directly from the '..\SteamApps\common\Baldurs Gate 3\bin' folder, by right clicking the executable (bg3.exe for Vulkan, or bg3_dx11.exe) and running as administrator.

Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button. If you are using the GOG version, in the Galaxy client, select the game, then the settings icon at the top right, beside the Play button and under Manage Installation, select 'Verify / Repair'.

Try browsing to the 'C:\Users\<UserName>\AppData\Local\Larian Studios' folder and delete the 'Baldur's Gate 3' subfolder, if it exists. The easiest way to get there would be to copy %LocalAppData%\Larian Studios into the location bar in Explorer and hit Enter.

This folder contains the saved games, profiles and configuration files. Normally it would be best to rename this folder for troubleshooting purposes, but if you don't have any saved games that isn't an issue. Next, extract the replacement folder from this download into your '..\Documents\Larian Studios' folder, and see if that will let you start the game properly.

The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset and low audio quality, which you can change in the options (manually, or hit autodetect) if this lets you create a new profile, start a game and save/load ok.

Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and "use original boot configuration options" checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button, then click OK and reboot the computer. Run msconfig again to switch back to the normal boot configuration.

Try creating a new Windows user account with a standard My Documents folder and not stored in a One Drive folder, as Windows 10 likes to do. Switch to that account to test.

If that doesn't help, please contact Support with a dxdiag report. Press WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy to then attach to your email to support.