You don't need Roman numerals for this compound.
If I understand your question correctly, then you don't need to use Roman numerals to name this ionic compound.
In this case, beryllium,
The idea with Roman numerals is that they are used in the name of an ionic compound when the metal can exhibit multiple oxidation states.
In essence, Roman numerals are a way of distinguishing between various oxidation states in which a transition metal, for example, can exhibit.
Since beryllium is located in group 2, it can only exhibit a
Your ionic compound is formed when one beryllium cation,
Notice that the subscripts of the ions are equivalent to their charges (in absoltue value)
#"Be"_color(blue)(1)("HCO"_3)_color(red)(2) -> "Be"^color(red)(2+) + 2 * "HCO"_3""^color(blue)(1-)#
The name of this compound will thus be beryllium bicarbonate.