Is it illegal to play guitar in Tik Tok?

Whether playing or covering other people's songs infringes the copyright of others depends on the specific situation.

If a cover program is performed for general public welfare purposes, no fees are charged to the public, and no remuneration is paid to the performer, no remuneration may be paid to the copyright owner without permission. However, unauthorized use of other people's songs to participate in commercial competitions, as well as commercial purposes such as live TV and shooting MTV, will all involve infringement, including singing in bars and spreading singers' cover videos on the Internet.

If it is for personal learning and appreciation, it does not belong to infringement. But the network is spread, and if it is put on the website, it must be approved by the copyright owner.