Hello i have a question lovenox (enoxaparin) is unavailable in my country ... is it okey to use other lmwh or rivroxaban in preventive doses in covid 19 patients(high level of d-dimeres)? (and contraindications) and thank you ?

Follow your local guidelines. Always. Not all areas of the world are the same.

Recent studies showed no difference in mortality between prophylactic use of anticoagulant or not. However, if you want to use prophylactic anticoagulant, you can use other LMWH or direct oral anticoagulants if Lovenox is unavailabe. Contraindications of these are the same as the general contraindications.