"The eleventh letter is not here."

Translation: Jedenáctý dopis tady není.

[danda.panda]

Ten jedenáctý dopis tu není

Please add. Thank you :)

[VladaFu]

We would use "THAT eleventh letter" for this.

[hans.sraj]

The word 'letter' means 'písmeno' as well - might be confusing. Thank you!

[VladaFu]

Added as an accepted answer.