Howdy!
This error is generally caused when the game is not able to properly utilize the graphics card/drivers. We often see this occur after the Operating System is updated or drivers update. I’d recommend to try each step below:
Step 1) Goto your Documents folder select: Overwatch -> Settings
Open Settings (may be listed as Settings_v0).
Modify or add the following line: WindowMode = "1"
Retest. If the issue continues try step 2.
Step 2) Goto your Documents folder -> Select Overwatch -> Delete the Settings folder.
Retest. Let us know if the issue continues!