You need an embassy visa for Mongolia if you have a Tajikistan passport
Visa options for Mongolia
Visitor visa
Travelers need a visa to visit Mongolia for Business or Tourism. A paper or embassy visa is the only visa available for travelers visiting Mongolia. The visa is typically a stamp or sticker added to the passport. Travelers must obtain this visa from a local embassy, consulate or visa center before traveling.
This is not offered by sherpa°.
Apply on the government site.