Registration of any .pso.tld domain fails.
Although .pso second level domain name is reserved by ICANN, the restriction affects only several TLDs. Nevertheless, OBAS considers .pso SLD as restricted for all TLDs.
Such behavior was classified as the software issue PBAS-29704 that was fixed in OBAS 4.5.5.
To resolve the issue, please upgrade to 4.5.5 as described in How to upgrade OBAS system to the latest available version.