Croatia became part of the European Union on 1 July 2013. The UK Government restricted access to employment in the UK for Croatian nationals for five years from this date, by requiring those who wanted to work to obtain authorisation (unless exempt) before starting work. From 1 July 2018 there are no longer any restrictions. Therefore Croatian nationals can work without first obtaining permission from this date.
If you were someone who was required to obtain an accession worker authorisation document under the 'work authorisation scheme', it is important that you keep the document (s) you were issued. If you applied for a document before 1 July 2018, it is important that application is processed and that you get this evidence (issued as a certificate) of your permission to work before 1 July 2018.