Hello @CDATIRE
I reproduced your steps and I was able to create VNET and MySQL Flexible server in WEST EU region
Note that the virtual network and subnet should be in the same region and subscription as your flexible server, so only virtual networks and subnets in the same region and subscription will be listed in the drop down.
Try to recreate your VNET and subnets
https://learn.microsoft.com/en-us/azure/mysql/flexible-server/how-to-manage-virtual-network-portal