You can run the sensors command on the terminal to check the CPU temperature. It should be installed by default. I tested it on Debian9 and Ubuntu20 and it worked without any error.
$ sensorsBAT1-acpi-0Adapter: ACPI interfacein0: 12.20 V coretemp-isa-0000Adapter: ISA adapterPackage id 0: +75.0°C (high = +84.0°C, crit = +100.0°C)Core 0: +75.0°C (high = +84.0°C, crit = +100.0°C)Core 1: +66.0°C (high = +84.0°C, crit = +100.0°C)Core 2: +70.0°C (high = +84.0°C, crit = +100.0°C)Core 3: +64.0°C (high = +84.0°C, crit = +100.0°C)acpitz-acpi-0Adapter: ACPI interfacetemp1: +69.0°C (crit = +127.0°C)
$ sensors
BAT1-acpi-0Adapter: ACPI interfacein0: 12.20 V coretemp-isa-0000Adapter: ISA adapterPackage id 0: +75.0°C (high = +84.0°C, crit = +100.0°C)Core 0: +75.0°C (high = +84.0°C, crit = +100.0°C)Core 1: +66.0°C (high = +84.0°C, crit = +100.0°C)Core 2: +70.0°C (high = +84.0°C, crit = +100.0°C)Core 3: +64.0°C (high = +84.0°C, crit = +100.0°C)acpitz-acpi-0Adapter: ACPI interfacetemp1: +69.0°C (crit = +127.0°C)