Can an american citizen travel to serbia right now ? I seen that all restrictions have been lifted ....can I fly now ?

No, thry've relaxed travel restrictions from some countries but the US is not included in them.

Serbia lifted the restrictions on air travel to/from the USA on June 6th.  It is apparently no longer a requirement, but get tested as close to departure as practical so you have a result showing you are negative for the disease. 

Also check the countries you might transit:

https://www.aljazeera.com/news/2020/06/coronavirus... 

Many European countries have opened to other European tourists, but not to people from the US, since we have not brought our virus infections under control. You need to google the specific  rules for entering Serbia. You can thank the people in the US who refuse to take precautions for our continuing rate of infection and our continuing inability to travel. 

If you want.  I wouldn't until the coronavirus is under control, but it is your choice.