Today I asked one question in the forumm.
I wanted to create a new tag in that question but I used my phone and the mobile version did not allow me to create one.
Is this a bug or a lack of functionality ?
To quote from Meta.SE:
Probably because it's really easy to make a typo when using a mobile browser, and they don't want to clutter the tag-space with misspellings, so it's prompting you to recheck.
For legitimate cases, just edit the tag in later when you're not using a mobile browser or switch to the full-site version.
This is the case. It's very rare that a new tag is intentionally and correctly created, and mobile device auto correct and difficulty typing makes it even more likely that tags are accidentally created. We just disallowed it altogether accordingly.