Anne,
The current requirement is that you have a PCR_RT covid test performed within 72 hours prior to entry. The option to fly directly to Managua is there but most people fly to Liberia Costa Rica and cross the southern border to Nicaragua. The airlines serving Managua (Avianca, Copa and AeroMexico) are not convenient from Europe. You will be much better off flying to Liberia Costa Rica take a bus or cab to the border, cross and take another cab or bus to your destination in Nicaragua.
I know several good taxi drivers in Nicaragua who could pick you up at the border. There are also companies with vehicles in both countries that pick you up at the airport drive you to the border and have another person in Nicaragua take you to your destination in Nicaragua.
I live in SJDS with my wife and have contacts for these services. You can also research on FACEBOOK and type in San Juan del sur and find multiple °µÍø½ûÇø groups with travel, taxi information. When are you planning on coming to Nicaragua? What part of the country are you planning to locate too? Feel free to contact me for any additional information.