Hi @Cha09, I am sorry to hear about this.
Your question has been translated to English as this is an International forum, please post any replies in English and it will be easier for me and other users to assist you.
Firstly, Xperia XA1 uses Nano Sim cards, so make sure that you use a punched out Nano SIM card. If you have converted your Mini SIM card to a Micro SIM by stamping it out the SIM card might be defective due to this process.
Secondly I would suggest that you check whether you SIM card is mounted correctly in your phone. For a detailed instruction on how to insert SIM card in your phone, I suggest that you check the user manual. The way to insert your SIM card differs a bit depending on if you have a Dual SIM-device or a Single SIM-device.
Another good idea is to check if your phone works with another SIM card. If your phone manages to detect another SIM card, the SIM card that you are using might be defective and in need of replacement. For example, older SIM cards used different voltages and if your card is older than 4-5 years I would suggest that you get in contact with your service provider in order to get a new SIM card.
Also, if you are using a SIM card that is brand new, I suggest that you make sure that your SIM card is activated. If your SIM card is new it might take a while until you can use it in your Xperia device. Contact your service provider if you are not sure if your SIM card has been activated or not.
Try these measures and let me know how it goes! 
0online
1898682posts