About the allocation of Elastic IP. Firstly it showed me that it could not be allocated "us-east-1c" but now its working

The field where you entered "us-east-1c" is namely Network Border Group. A network border group is a unique set of Availability Zones, Local Zones, or Wavelength Zones from which AWS advertises IP addresses.

For more information about Availability Zones, Local Zones, and Wavelength Zones, see Regions and zones in the Amazon Elastic Compute Cloud User Guide.

You may further refer Elastic IP address concepts and rules. You either allocate an Elastic IP address from either Amazon’s pool of public IPv4 addresses or from a custom IP address pool that you bring to your AWS account.

The IP addresses which can be associated to your resource hosted in us-east-1c Availability Zone falls under us-east-1 network border group. Similarly, us-east-1a, us-east-1b, us-east-1d, us-east-1e, us-east-1f also falls under the same. Hence, the correct option is us-east-1 as a result it worked for you.