Howdy,
Does your left shift key work otherwise? I like to test keys with the Keyboard Viewer.
From If your Apple wireless mouse, keyboard, or trackpad isn't working with your Mac as expected
Your keyboard has one or more keys that don't respond
Use the Keyboard Viewer to test whether the keyboard keys are responding correctly when they are pressed.
- Choose Apple menu > System Preferences, then click Keyboard.
- Click the Input Sources tab.
- Make sure that the keyboard language that you use is listed on the left side. If it's not, click the add button (+) and choose from the languages that appear.
- Select the ”Show Input menu in menu bar” checkbox.
- Click the Keyboard tab, then select ”Show keyboard and emoji viewers in menu bar” checkbox, if present.
- From the Input menu in the menu bar, choose Show Keyboard Viewer. The Keyboard Viewer showing the keyboard layout appears on your display.
- Type the key on the keyboard that doesn’t respond and see if the corresponding key highlights on the Keyboard Viewer. If it does, that key is functioning correctly.
If you enable the Mouse Keys feature, many keys might not respond as you expect. To turn off Mouse keys, choose Apple menu > System Preferences, then click Accessibility. Choose Mouse & Trackpad from the list on the left side, then then deselect the Enable Mouse Keys checkbox.
ivan