hi fariasol, and welcome to TikiWiki Community!
Please, ensure two things:
(1) that you have at least 32 Mb in your php.ini for the memory_limit param (see Requirements and Setup,
You might need to execute:
sudo gedit /etc/php5/apache2/php.ini
or in similar place, if in your distro it's in another place.
Search and edit (if needed) where it says:
memory_limit = 8Mb
memory_limit = 32Mb
(2) And check also that you have all the necessary packets for your ubuntu installed.
See this list:
If you think that a page in the Documentation explaining this package list for ubuntu is kind of needed for new users, please go ahead and create that page in doc.tw.o