Issues creating new Lightsail instances

Hello.

Your Lightsail instance's quota may be set to a lower value than the default value of 20.
Even if the quota screen shows 20, the actual number may be lower.
Therefore, please try applying for quota upper limit relaxation from the service quota screen.
https://docs.aws.amazon.com/general/latest/gr/lightsail.html

New AWS accounts might start with quotas that are lower than those described here.

You can apply from the URL below.
https://console.aws.amazon.com/servicequotas/home/services/lightsail/quotas/L-4259AF9B

Is your account particularly new, like 1 or 2 days old? If it is then it's possible there might be a temporary cap on the number of instances that you can create, just while your account is fully initialised and billing methods are validated, etc.

If your account is long-standing and you're seeing this error your best course of action is to uplift the Lightsail quota to 20. But if your LIghtsail quota is already at 20 then consider logging a billing support case https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-get-answers.html#billing-support

This scenario looks similar to yours, and a billing support case was required to resolve it https://stackoverflow.com/questions/64169732/sorry-youve-reached-your-maximum-limit-of-lightsail-instances-2