Your Bluetti EB1500 requires a minimum of 16 volts incoming to begin charging. Even though you measure 20 volts from the panel when not connected, the voltage from the panel is most likely dropping below the 16 volt minim when under a charging load. You can purchase a higher voltage panel or purchase and identical panel and connect them in series connection which will double the volatage and allow charging to begin.
Any solar panel will list its open circuit voltage which is max power with no load connected and will also list the voltage at maximum load which will be lower. Not sure what panel you have or what the specs are but in any case, two panels should solve your charging issue as well as having double the wattage available for charging.