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