Error: Could not find or load main class com.intel.bts.BtsApp

Have you tried the suggested fixes?
for me, under ubuntu 18.04 worked after

installing Oracle Java as said by @JRose and setting proper java-alternative option

installing openjfx as said by @rliston

this almost made it work, it still failed with following errors

user@host:~/workspace_local/board_test_system$ java --module-path /usr/share/openjfx/lib --add-modules javafx.base,javafx.controls,javafx.fxml,javafx.graphics,javafx.media,javafx.swing,javafx.web -Xmx256m -jar bts.jar -mode pwr
Graphics Device initialization failed for : es2, sw
Error initializing QuantumRenderer: no suitable pipeline found

I was desperate, and I found that I had the missing class from a Xilinx installation folder...so I decided to try.

I know it may have "exploded", but for me, it made the trick, and I could take the power measures.

user@host:~/workspace_local/board_test_system$ java --module-path /usr/share/openjfx/lib --add-modules javafx.base,javafx.controls,javafx.fxml,javafx.graphics,javafx.media,javafx.swing,javafx.web --module-path /opt/Xilinx/.xinstall/DocNav/tps/lnx64/javafx-sdk-11.0.2/lib/ -Xmx256m -jar bts.jar -mode pwr
Success!
Starting application on Linux...
Jun 10, 2022 4:28:27 PM com.intel.bts.ClientApp attachServer
INFO: [/opt/intelFPGA_pro/20.1/quartus/sopc_builder/bin/system-console, --server]
Jun 10, 2022 4:28:27 PM com.intel.bts.ClientApp attachServer
INFO: Here is the standard output of the command:
Jun 10, 2022 4:28:37 PM com.intel.bts.ClientApp attachServer
INFO: TCP PORT: 42045
Jun 10, 2022 4:28:44 PM com.intel.bts.JtagInfo createDeviceInfoMapArray
INFO: 2 device detected: {/devices/1SM21BHN(1|2|3)|1SM21BHU1|..@1#1-11#Intel Stratix 10 MX FPGA Development Kit} {/devices/VTAP10@2#1-11#Intel Stratix 10 MX FPGA Development Kit}
Jun 10, 2022 4:28:44 PM com.intel.bts.ClientApp getMatchedBoardInfoArray
INFO: System Max matched: true, Fpga matched: true
Jun 10, 2022 4:28:44 PM com.intel.bts.BtsView startMainStage
INFO: Selected JTAG cable is: Intel Stratix 10 MX FPGA Development Kit on localhost (1-11)
Jun 10, 2022 4:28:44 PM com.intel.bts.BtsView startMainStage
INFO: System MAX on index 2, FPGA device on index 0
Jun 10, 2022 4:28:44 PM com.intel.bts.SysConService <init>
INFO: Claim master service succeed! Service path: "/devices/VTAP10@2#1-11#Intel Stratix 10 MX FPGA Development Kit/(link)/JTAG/(110:132 v1 #0)/phy_0/master", claimed path: /channels/remote1/(lib)/master_1.
Jun 10, 2022 4:28:44 PM com.intel.bts.InstanceChecker registerInstance
INFO: Application Register. Type: Power or Clock GUI
Jun 10, 2022 4:28:44 PM com.intel.bts.SysConService <init>
INFO: Claim master service succeed! Service path: "/devices/VTAP10@2#1-11#Intel Stratix 10 MX FPGA Development Kit/(link)/JTAG/(110:132 v1 #0)/phy_0/master", claimed path: /channels/remote1/(lib)/master_2.
Jun 10, 2022 4:28:45 PM com.intel.bts.SysConService <init>
INFO: Claim master service succeed! Service path: "/devices/VTAP10@2#1-11#Intel Stratix 10 MX FPGA Development Kit/(link)/JTAG/(110:132 v1 #0)/phy_0/master", claimed path: /channels/remote1/(lib)/master_3.
Jun 10, 2022 4:28:45 PM com.intel.bts.SysConService <init>
INFO: Claim master service succeed! Service path: "/devices/VTAP10@2#1-11#Intel Stratix 10 MX FPGA Development Kit/(link)/JTAG/(110:132 v1 #0)/phy_0/master", claimed path: /channels/remote1/(lib)/master_4.
Jun 10, 2022 4:28:45 PM com.intel.bts.PowerDriver setLtc3884IinCalGain
INFO: DCR of IIN:0.5

And the Power Monitor finally appeared !!