Hi, Cosme!
This is a common mistake in Spanish... When you use the structure
deber + infinitivo, you are always indicating an
obligation. This confusion arises out of this other structure
deber + de + infinitivo, which refers to a
probability: e.g.:
debe de tener ochenta años (he/she must be eighty). Therefore, in your example, the correct option would be to use the first structure.
With respect to the other question, "
decimosexto" would be the correct word for 16th in Spanish. Note that it is written without
a "tilde", and remember to include a "tilde" in "
décimo"
Hope you find it useful!