Hi @evonmaxilom
Use a DMM (digital multimeter) to check if there's 20V DC on the output plug of the charger. If you don't have a DMM then try using a another known working, compatible charger and check it works OK when charging the laptop.
If the charger is OK then here's the [https://download.lenovo.com/pccbbs/mobiles_pdf/t450_hmm_en.pdf|hardware maintenance manual] for the laptop.
Go to p.81 to view the procedure to remove the DC-In cable. At this stage all you need to do is to unplug the cable from the motherboard and then use a DMM to check if there's 20V DC when measuring between the red and black wires on the cable plug, with the charger connected to the jack as per normal.
''If there is no voltage detected'' then the jack is faulty and it will need to be replaced as it cannot be repaired. Search online for ''00HN539'' to find suppliers that suit you best.
''If the correct voltage detected,'' then the [https://elektrotanya.com/lenovo_thinkpad_t450_nm-a251_uma_nm-a252_aivl0_rev1.0_sch.pdf/download.html#dl|schematics] for the motherboard would be required to find out what's wrong.