All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad (sallallaahu 'alayhi wa sallam) is His slave and Messenger.
The phrase of tawḥīd, “Lā ilāha illā Allāh”, consists entirely of letters that are easy to pronounce—even for a non-Arab, such as an English speaker—since it contains no letter that is difficult to articulate, like the letter ḍād, for example.
If an error does occur while you are pronouncing it, then it may have been due to rushing while repeating it. This can be avoided by reciting it without haste.
As for the ḥadīth you are referring to, it concerns the difficulty and faltering that arise from weakness in memorization. An-Nawawī
said in his commentary on Ṣaḥīḥ Muslim: “It refers to the person who struggles and becomes unsure during recitation due to weak retention.” [End quote]
Similarly, At-Ṭībī
said in his commentary on al-Mishkāt: “That is, his recitation is marked by struggling and uncertainty, and his tongue becomes constrained and heavy because of weak memorization.” [End quote]
Allah knows best.