You may need an embassy visa for Serbia if you have a Hong Kong SAR passport
Visa options for Serbia
Visitor visa is required for trips longer than 14 days
Travellers need a visa to visit Serbia for Business or Tourism for more than 14 days. Travellers do not need a visa if staying for 14 days or less. Travellers must have a Hong Kong Special Administrative Region passport. A paper or embassy visa is the only visa available for travellers visiting Serbia for more than 14 days. The visa is typically a stamp or sticker added to the passport. Travellers must obtain this visa from a local embassy, consulate or visa center before travelling.
This is not offered by sherpa°.
Apply on the government site.