You are not able to add an external member into a Space already created with external sharing enabled.
You have to provide the steps to the external user, which will allow them to create a Google account:
- Go to the Google sign in page.
- Click Create account.
- Enter your name.
- Click Use my current email address instead.
- Enter your current email address.
- Click Next.
- Verify your email address with the code sent to your existing email.
- Click Verify.
The external user does not have a Google account.