Go to Edit Server > Server Dashboard > On the main page for your server dashboard there should be a thing that says 'Command Bar'. Under that is a text box. In there, with your server started, type: minecraft:kill @e[type=item] then restart your server. If it doesn't work I have an alternative method.
Download ClearLag and Skript plugin.
Restart. Once both are installed go into File Manager > Plugins > Skript > scripts > Create a new file named r.sk
In there type this exact code, even copy and paste it.
on server start:
loop 3 times:
execute console command "/minecraft:kill @e[type=item]"
Hit the 'Save' button, restart your server, should work just fine.