I've finally completed step 4. Test automation logic. 

It took me long, because of incorrect requirement in spec. 
The requirement is: 
When an existing maintenance request of type Repair or Routine Maintenance is Closed, you create a new maintenance request for a future routine checkup. [...] The Subject should not be null and [...]

Unfortunatelly test checks something completelly different. See below line from error log:  
Execute Anonymous: System.assertEquals(402, [select count() from Case where subject like '%AMC Spirit%']);
Moreover, the error information from within Trailhead page suggests, that this is a problem with bulk processing. 

<joke> So, if you want to finish step 4 of those badge you need to set the subject to AMC Spirit in case created by your trigger </joke>


 

I've finally completed step 4. Test automation logic. 

It took me long, because of incorrect requirement in spec. 
The requirement is: 
When an existing maintenance request of type Repair or Routine Maintenance is Closed, you create a new maintenance request for a future routine checkup. [...] The Subject should not be null and [...]

Unfortunatelly test checks something completelly different. See below line from error log:  
Execute Anonymous: System.assertEquals(402, [select count() from Case where subject like '%AMC Spirit%']);
Moreover, the error information from within Trailhead page suggests, that this is a problem with bulk processing. 

<joke> So, if you want to finish step 4 of those badge you need to set the subject to AMC Spirit in case created by your trigger </joke>