Failed to re-start the existing EC2 instance due to vCPU limit

If the instance you are starting only has 1 vCPU, then you probably have another instance already running, consuming all the capacity your current quota allows. You can either stop that other instance or request AWS to increase the quota, as the error message advises.

Those quotas exist to protect you against costs unintentionally getting out of control. When you're having to request a quota increase explicitly, it won't happen as easily and by accident as if you could run unlimited numbers of instances without necessarily realising each of them costs money (when exceeding any potential free tier limits that might apply, that is).

Temporary Workaround (If Immediate Solution Needed):

Stop or terminate other running instances that might be consuming your vCPU limit to free up capacity. Evaluate your instance usage and consider resizing or optimizing instances to better fit your workload within existing limits.

also you could change the tier of the EC2 for example from t3.m to t3a.m , worked for me. hope, it helps you