If the lid fits well and the bellbox is fairly new, chances are it is a fault with the wiring, or an ill-fitting PIR lid.
You could confirm a tamper fault to the wiring by disconnecting the two wires from the TAMP terminals and replacing them with a short piece of wire to simulate a closed loop (effectively what a healthy closed tamper circuit is).
If when the panel is closed up the tamper light goes out and the system allows arming, you have found your fault. You will then have to use a multimeter on continuity to test the tamper loop to find the break.
If fitting a loop to the TAMP terminals does not solve the problem, then you will have an issue with the lid tamper on the panel or the bell box tamper.
You may find that you have several pairs of wires joined together to form a series loop. Each pair of tamper wires in each cable represents the tamper loop to that area, zone or individual accessory.
In which case you could check each individual cable's tamper loop to pinpoint the problem.
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