This information is constantly changing, so it is best to follow up with the airlines and Turkish authorities for what documents are needed. When we travelled, we needed to present a return/onward flight ticket, a valid Turkish visa and a negative covid test taken at least 72 hours before our flight even though Turkey accepted a certificate of full vaccination without a PCR or antigen test. In addition, we filled out an online health form for both Kenya and Turkey. You can check out this post for all the details.