Yani,

If your design pressure conditions include -0.21 barg then that would appear to be the negative gauge pressure range (i.e. 0 to -1 barg). In that case your design external pressure for the purposes of calculation would be 0.21 barg.

By comparison, if the design pressure was specified as 0.21 bar absolute
(the atmospheric pressure outside would be 1 bar absolute) this would result in approx 0.79 bar external pressure.