Dear @Petrus,
Thank you for your reply.
A so-called "Fiktionsbescheinigung" is issued to people who are staying in Germany and have applied for the issuance or extension of a residence permit or settlement permit if the immigration authorities cannot or do not want to make a decision straight away.
Legally, a fictitious certificate is valid as much as the residence permit you previously held. If I remember correctly, some German federal states issued "Fiktionsbescheinigungen" to third country nationals who previously have lived in Ukraine and might meet (soon) the requirements for residence permits according to section 16a (vocational training/Ausbildung) or 16b (studying) of the German Residence Act. But I don't have any information on the respevctive procedures until the actual issuing of a residence permit. Holding a residence permit according to 16a, you might be able to switch your "Ausbildung". Please see this thread and Handbook Germany for further details. However, since I am not familiar with your individual case (and neither a lawyer), I highly recommend to consult a migration counselling office for more tailored advice.
I hope this helps and all the best,
Meike